Refactor the TestRebase integration test

- Create open_pr() method in SandboxTest
- Test name and owner of rebased PR
Add simple scenarios to test latest merge options (shallow and remote)
Refactor filter commands input parsing unit tests
Test organization default filter
Add unit test for SetCommitStatus command

- Create abstract UnitTestFilteredPullRequestCommand class from which the
  Merge and SetCommitStatus unit tests classes inherit
- Add statuses tests to SetCommitStatus unit test class
- Add base test to TravisMerge unit test class
Add SetCommitStatus unit tests to travis yml
Refactor TestDeploy unit tests
Add test for token precedence in get_token_or_user()
Use mixin class for UnitTestMerge and UnitTestSetCommitStatus

This commit allows to group tests into without running into errors while
unittest tries to instantiate an abstract class (see also
http://bugs.python.org/issue17519)
Merge pull request #87 from sbesson/integration_test_refactor

Refactor the TestRebase integration test