Travis CI integration

This pull request adds a .travis.yml file that builds r2 and runs the test suite under AddressSanitizer, MemorySanitizer and ThreadSanitizer. Valgrind isn't yet run.

Travis can easily be enabled for pull requests, I don't know how well that works with Jenkins.

Preliminary results can be viewed at https://travis-ci.org/neuschaefer/radare2/builds/27454564.