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.

Thanks to @dvliman and @kxepal for noticing.

Floating point numbers are no longer encoded as a one to one mapping
of their binary representation, but as short as possible (while still
being acurate). The double-conversion library [1] is used to do the
hard work.

The ECMAScript compatible conversion is used.

[1] https://code.google.com/p/double-conversion/