Allow real names in addition to usernames for --cc, not just --rr

Summary:
I also allow a trailing @ to turn off the real-name lookup.

Get rid of a debugging statement that got in somehow.

Lint didn't catch this because 'arc' doesn't end in .py. :-(

Add in the uniqification that the old code had.

This probably isn't necessary in practice, but why regress?

Add a few improvements based on code review feedback.

Mostly, we now properly discard empty arguments for --rr:
   arc diff --rr foo,,bar,,,,,baz

Test Plan:
When I send this for review, I will do
   arc mail --rr chris@ --cc ben
and make sure I get the chooser to pick a ben.

None.

Fingers crossed.

Fingers crossed.

Reviewers: chris

Reviewed By: chris

CC: benkomalo

Differential Revision: http://phabricator.khanacademy.org/D2844