Remove some old cruft from rebar.config

I must've originally copied this config from Emonk.
Encode floating point numbers as short as possible

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/
Check in the double-conversion dependency

While using rebar's dependency management was surprisingly easy it
broke apps that tried to build Jiffy as a dependency due to relative
path #includes.

This also fixes a few other issues. Most notably it removes the use of
the ECMAScript compatible encoding due to JSON's lack of support for +/-
Inf and NaN.
Bug fix for large doubles with a fraction

My truth table was a bit busted for extreme value doubles.
Add tests from double-conversion library

I pulled in a lot of tests from the double-conversion test suite to make
sure we don't accidentally break behavior in the future.
Forgot to refactor the roundtrip tests

I removed some columns to try and minimize the amount of test data I'm
requiring and forgot to update the test code.
Turn on compiler optimizations

Because reasons.
Minor change for building on Windows
Fix compilation on GCC 4.8