Add support for rebar3

Allow Jiffy to be used easily in projects using either rebar 2 or 3.

PropEr broke my support for R14. Turns out that EQC Mini is quite usable
so I've just switched to that. If EQC Mini exists it will be used, if
not the test is skipped gracefully.

Originally reported by @NorthNick on apache/couchdb-jiffy.

Fixed wrong jiffy:decode/2 spec

Previously if a key was malformed UTF-8 and the user specified the
`force_utf8` option we would fail to try and encode a fixed up version
of the object. This was due to missing a clause to catch the
`invalid_object_member_key` exception. This adds the clause and a couple
tests to ensure it works.

This was marked in the c_src/double-conversion directory but I've gone
ahead and added a copy to the root LICENSE file.

Time for Jiffy to enter the world of Erlang package management.

Destroy map iterators once we're done with them.

Each call to enif_map_iterator_create() must be paired with a call to
enif_map_iterator_destroy(). Otherwise, we'll leak memory.

Fixes #112

rebar.config: set CFLAGS/CXXFLAGS/LDFLAGS on Hurd

The environment variable approach breaks other projects' Travis-CI runs
if they include other projects that depend on a different version of
PropEr.

Fixes: #106

Use the common CFLAGS/CXXFLAGS/LDFLAGS used on Linux/FreeBSD/etc also
on GNU/Hurd.

Fix win32 build

This test has proved its point and now just exists to randomly fail some
builds depending on race conditions with the garbage collector.

I was being lazy and relying on Travis. This fixes up the map spec for
newer VMs.

This obviously only exists on VMs that support it.

In the original PR for `return_trailer` @vlm pointed out that I wasn't
using enif_consume_timeslice correctly. This fixes that by changing out
its called.

Previously we attempted to define the total number of bytes to decode or
encode in a single NIF call and then would consume as much of the
timeslice as we processed. This is wrong because we may start the NIF
call with less than an entire timeslice left.

The new approach is to define the number of bytes to encode or decode
per reduction and then iteratively call enif_consume_timeslice until it
indicates that we should return.

Previously Jiffy would throw an error about trailing data if there is
any non-whitespace character encounter after the first term had been
decoded.

This patch adds a decoder option `return_trailer` that will instead
return a sub-binary starting at the first non-whitespace character. This
allows users to be able to decode multiple terms from a single iodata()
term.

Thanks to @vlm for the original patch.

Add compile option no_native

Thanks to @joedevivo for the contribution.

Fixes #85

Thanks to @dvliman and @kxepal for noticing.

Add replace_undefined and escape_forward_slashes

This brings back escaping forward slashes as an option during encoding.
Default is still not to escape.

This is pretty much a generalisation of the use_nil option to support an
arbitrary atom.

Changed pos to unsigned int to prevent warning from happening.

update README.md fixing typo in maps example

rebar.config: fix Windows support (CFLAGS, CXXFLAGS)

This sounds rather insane to me but I've managed to show that `(char)
-1` is converted to 255 on some platforms. This was reproduced on
ppc64el via Qemu on OS X. A simple program that does `fprintf(stderr,
"%d\r\n", (char) -1);` prints 255 to the console. Rather than rely on
the signedness of a char I've just updated things to use an unsigned
char (which hopefully is never signed) and replaced -1 with 255 for the
sentinel value when converting hex values.

Thanks to Balint Reczey (@rbalint) for the report.

Fixes #74

This fixes a leak when encoding a bare bignum. Technically it would be
possible to hit this memory leak randomly with bignums in objects but
the chances are highly unlikely.

Thanks to @miriampena for the issue.

Fixes #69