Add AlreadyMerged command

`scc already-merged` now provides a way to check which
branches both local and remote can be removed. Anything
not listed should be inspected for removal.
Systematically test the Github connection after instantiation

This additional checkpoint allows any scc command to fail-fast if bad
connection credentials are passed.
Add tests for failing Github connections
Add Github connection tests to Travis build
Change TestGithub superclass to unittest.TestCase
Add Label command
Improve exception handling
Merge pull request #29 from joshmoore/add_labels

Add Label command
Add CheckMilestone command

This determines PRs between a range and
then checks whether or not they have a
milestone set.
Suggested improvements from Sebastien

 * Catch 404 on invalid permissions
 * Include PR in message
Merge pull request #43 from joshmoore/in-milestone

Add CheckMilestone command
Merge pull request #33 from joshmoore/already-merged

Add AlreadyMerged command
Merge pull request #46 from sbesson/bad_credentials

Fail-fast if bad credentials are used