Add travis-merge command

Add a new command similar to the Merge class where all the default merge
parameters (filters, base branch) should be read from environment variables
and or Pull Request comments for automatic testing using Travis CI.
Add the travis-merge command to .travis.yml
Do not ask for the login or token in a Travis environment
Fix value returned by Travis if job is not a Pull Request
Refactor test_redirectories() function as parse_comments()

- fix usage of test_directories()
- parse included PRs (by number) in travis-merge using "--depends-on"
Construct PullRequest object to parse comments in travis-merge
Fix include filter initialization
Fix dependency inclusion logic
Raise zero error code if TRAVIS_PULL_REQUEST is 'false'

Allow Travis build on individual branches to be successful
Merge pull request #77 from sbesson/travis_scc

Add support for scc merging in Travis