Add .hrl for -include_lib support

I can include libraries like [proper](https://github.com/manopapad/proper) and [getopt](https://github.com/jcomellas/getopt) by linking into my Erlang libs folder and inserting `-include_lib("../.hrl").` in my code. I tried this with mochijson2, but I can't seem to find a `.hrl` file in the mochjson2 codebase.
