Running "make eunit" will now download proper into deps and ensure it
is on the code path. Projects that depend on jiffy will no longer pick
up a transitive dependency on proper.

When the eunit target is run, proper will be cloned into deps/ if not
already found and compiled (also if not already compiled).

Also includes a few Makefile tweaks:

* Add distclean target
* Add .PHONY for Makefile target hygeine

* 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.