jiffy:encode/2 now accepts a 'pretty' option that will format the JSON
output using two-space indents.

I need to fix this so that object sizes don't explode when generating
larger values. Basically, as the type generator recurses is should be
adjusting the size value.

Not only was I not actually sending atoms to Jiffy, the test will end up
exhausting the atom table and crashing the Erlang VM.

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