Added R16b support

Hi, 

The `edis_gb_trees` module extends `gb_trees` with the use of an `-extends` directive. 

This commit adds support for Erlang/OTP R16 thanks to the `pmod_pt` parse tranform. I've been able to test using `ZADD`, `ZRANGE`, etc. commands.

As most of you already now, this parse_transform handles `-extends` directive with `$handle_undefined_function`.