build: Add continuous integration support via travis-ci

This allows any libnl github repo to link to travis-ci which
will result in new commits and pull requests to be automatically
built with both gcc and clang. Any build failures including the
addition of warnings are reported to the author and committer.

Following Thomas Graf's suggestion:
https://plus.google.com/103961622207897185587/posts/GaVC6jwcMgp

Signed-off-by: Tobias Klauser <tklauser@distanz.ch>