[WIP]Adding support for Mercurial repositories to RB-Gateway.

Review Request #9398 — Created Nov. 25, 2017 and discarded — Latest diff uploaded

rb-gateway, students

Added the hg case to LoadConfig, this allows Hg repos to be defined
in the config file. The HgRepository type has been created and
currently supports GetName, GetPath, GetFile & GetFileByCommit.
Interactions with Hg repos are done through Mercurial's command server
through gohg.

httpie was used to send GetName, GetPath, GetFile &
GetFileByCommit. GET requests requesting information from an Hg repo.
Each request yielded the expected result.