Add travis support

Add integration with free CI service http://travis-ci.org to execute unit-tests on each push/branch/merge-proposal.
At the moment nih_watch_reader tests hang indefinitely on travis-ci infrastructure which have been disabled for now.

I will investigate further why it fails in their environment. To further gather environment details I've executed procenv in that environment (expand context to see line 450 onwards at https://travis-ci.org/xnox/libnih/builds/14676025 ) which indicates an Ubuntu 12.04 OpenVZ container.