Add a clang-sanitizers test-build in the CI.

[LefterisJP]

Integrate clang-sanitizer in a special build and add it to Travis.

Even though we currently have valgrind run in Travis it would be nice to also have clang's sanitizers. Add the following sanitizers:

• address
• memory
• undefined behaviour
• memory leaks

Optionally add in the future but not needed now:

• thread sanitizer. At the moment no threads are used so this is not needed yet.
• data flow. To check conformance to an ABI we would specify. Also not needed.