Add pin_id to post response for Pins endpoint

Added pin_id to response from POST against pins endpoint. Also added id
being returned in the pins regular GET response. This is to help
facilitate comments -> pins and pins -> comments UI connections.

@popwarfour You should be able to start playing with linking pins and
comments together on iOS now.
Updated readme with contribution section

Added contribution section for the readme and removed Colin from the
project as requested.
Removed random folder.

Don't know where this folder came from, I could go a git blame and
bother whoever's fault it is that these random folders came into
existence; but I'll hold off.

@popwarfour I'm just going some cleaning of the repository and there's a
bunch of folders in the ios directory and there's one GoGreen folder in
the root level of the repository. WHICH of these is the build you're
using, and please use git rm to remove the other ones and have the ios
project be just in the ios directory. Thanks.
Merge branch 'develop' of github.com:EJEHardenberg/GreenUp into develop