Just trying to be a bit more thorough in the Makefile targets.

This runs a `git clean -fxd` to revert to a pristine Git clone.

* Update rebar to a more recent version
  * Update rebar.config for deprecation warnings
  * Allow people to export the REBAR environment variable to use
    their own version of rebar
  * Fixed `make clean`

Big thanks to Dmitry Groshev [1] for help getting PropEr tests into
Jiffy. These generate valid EJSON to check parsing as well as check
that random inputs don't cause segfaults or other nasty effects.

Future improvements would be to write a JSON generator as well as a
version that introduces known errors into the binary for checking known
parsing errors as well.

[1] https://github.com/si14

Fixes: #10

Passes all eep0018 tests.