In OTP-17.0 paths on windows are not shortpaths anymore
so they may contain spaces and parenthesis

This commit fixes issue #98 by removing the ERL_LIBS and REBAR_DEPS_DIR
from the list of environment variables exported by the rebar_port_compiler
plugin.

REBAR_DEPS_DIR is often needed when a nif needs to be linked with a raw
dependency.

* allow plugins to print help message for implemented commands
* append core rebar.config options to common 'rebar help' message

The results returned by os:getenv() may contain unicode characters.
That said, we need to explicitly allow unicode when splitting the
environment information, otherwise badarg will be raised causing all
rebar commands to fail until the environment variable is removed.

- compatible with rebar 2.0.0

{port_specs, [{".*", "priv/foo.so", ["c_src/foo.c"], [{env, []}]}]}.

* consolidate options
 * add support for building executables

Previously if erl_interface could not be found {error, not_found} was
incorrectly used as a path. With this change we fail early with a
descriptive error message.

This patch adds support for first_files to the port_compiler, via the
new `port_first_files` rebar config element.

The introduction of setup_env as a global concept caused the rebar_port_compiler
implementation to start getting called a LOT. The expansion of environment variables
that happens in the port compiler was O(n^n), which means you could see upwards of
80k invocations of lists:foldl on a single app "./rebar clean". This commit reworks
the expansion to be O(n^2), and reduces the running time for the same operation by
60%+. On a large project like Riak, the end result is that a build went from 200
seconds to 73.

If MSYS (with bash) is not installed on Windows then do the shell
variable substitution by ourselves. Otherwise just call bash to do the
job.

- Port compiler no longer requires bash on windows.
- It's possible to use compilers whose command lines don't fit into
  the default template

Calling erlang:system_info(wordsize) yields the internal word size of
the Erlang emulator. But due to the halfword emulator, need to pass
{wordsize, external} instead to get the word size, or pointer size, as
seen by external code such as NIFs. The halfword emulator has 4 byte
internal words but 8 byte external words due to 64-bit compilation,
which means NIFs for the halfword emulator also have to be compiled
64-bit. But just passing wordsize is equivalent to passing {wordsize,
internal}, which does not indicate the pointer size for the halfword
emulator.

Older versions of Erlang do not support {wordsize, external}, though,
so continue to pass just wordsize for those versions.

Change the second parameter of a regex tagged port_source from being a
filename or wildcard to being a list of filenames or wildcards.

Previously: {"R14", "c_src/*.c"}
Now: {"R14", ["c_src/*.c"]}

Motivation for change is to avoid repeating regexes.

The logic changes from strict overrides to a hybrid for merging os_env
and defaults, whereby defaults are chosen if they are not set in the
os_env or if the defaults contains substitutions (at which point os
environment is substituted).

This still means that rebar.config works as it did before, where it
overrides or substitutes based on the use of $VAR, but that default
and os environment merging works such that the common cases of
providing CC or LDFLAGS in the users environment or command line work
as you might expect.  In that CC is overriden by the os environment
unless rebar.config overrides it, and LDFLAGS from the user environment
is appended or prepended based on defaults and rebar.config.

Addresses problems with https://github.com/basho/rebar/pull/71
Also keeps the fix for bug 255.