Integrate representors mime type registration

@fosrias @sheavalentine-mdsol @miwest929 @HonoreDB 

DO NOT MERGE YET!

This is initial commit integrating representors into crichton. 
It includes updated mime type registration. There are some problems with POST which I'm trying to resolve.

Also, travis can't access representors from github. Therefore build is failing. Someone should update keys over there.

Where is `include_private` used?