Continuous integration and test coverage

@alainjobart @sougou @enisoc @yaoshengzhe 

Integrated with Travis and Coveralls
- https://travis-ci.org/youtube/vitess
- https://coveralls.io/r/youtube/vitess

Integrations tests are grouped into 3 suites and this helps with parallelization of the builds.
All targets are retried up to 3 times before they get marked as failed. 
Skips a few tests that are too flaky even after the retries.
Have turned off email notifications for now, will turn this back on once merged into master.
Yes. As mentioned in the description above, will turn it on once this is in master.