Turn on compiler optimizations

Because reasons.

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.

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.

No longer escape forward slashes

After a few requests and some reflection I've decided to stop escaping
forward slashes in strings while still accepting the escaped version
through the decoder. This appears to mimic the behavior of other popular
JSON libraries I've checked (Ruby and Python).

Adjust rebar.config to support rebar v2.0

- Include Windows preprocessor definitions that rebar is missing
- Minor changes to optimisation flags

Make PropEr a soft dependency

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.

Fix code reloading for Jiffy

As it turns out I did not understand the documenation for
load/upgrade/unload correctly. load/upgrade are called conditionally if
there's code in the VM for the NIF. Ie, no code means load is called,
where as if code exists, upgrade is called.

unload is called regardless once per load/unload. This means that
load/upgrade in Jiffy's case will each create a state object and unload
will free it each time. I was missing the fact that unload is called
every time and hence I don't need to clear the state in upgrade.

Be paranoid about the values sent to upgrade

The docs aren't entirely clear on the order of calls for upgrades so
this is mostly just in case old_priv ever happens to not be what load
returned in priv.

Fix hot code upgrades

Fix compiler warning for gcc 4.4.3