Properly pick up the TEST_REPO_BASE_URL var in the scenario tests
Avoid pushd/popd in scenario tests, they are not always available
Add support for Travis CI

This commits adds a .travis.yml for Travis to be able to run
the python-consonant test suite.

Non-trivial additions to this file:

* The .travis-libgit.sh script builds a static version of libgit2
* Email notifications are currently sent to Jannis only
* TEST_REPO_BASE_URL is set to "git://github.com/CodethinkLabs"
  to allow Travis to clone the test repositories required for
  running the test suite
* The CFLAGS and LDFLAGS are required to get pygit2 to work