Add a "createcomment" method to Differential

Summary:
Added a new method differential.createcomment

Task ID: #752014

Test Plan:
I created a test diff and called this method via the conduit
from a client PHP script to add comments. I confirmed that

1. the comment appeared on the revision, 2) URLs within the

comment were turned into hyperlinks, and 3) Phabricator
sent a notification email to the people watching the test
diff.

Reviewers: nh, jungejason, epriestley

Reviewed By: nh

CC: aran, nh

Differential Revision: 1128