Use Travis-ci to run the tests

I've updated the makefile in the `tests` directory and added a config file for travis-ci. It runs the tests with clang3.3 and g++4.8 (it installs it first, because travis ci uses g++4.6 as default).

And I changed the return code of the `run_test_set` to return `true` if all tests of the test set pass, so the test runner returns 0 when all tests passed.

Cheers