This should hopefully allow users to use rebar3 to compile jiffy while
also not breaking any project still using rebar2.

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

This patch adds initial support for decoding/encoding to/from the new
maps data type.

I'd like to thank Jihyun Yu (yjh0502) for the initial versions of this
work.

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/

This prevents applications that depend on Jiffy from requiring PropEr as
a dependency just to run Jiffy's full test suite. If you want to run the
full test suite you'll have to run Jiffy's Makefile directly which
creates a `.jiffy.dev` marker that enables the full test suite.