Add a travis ci configuration

If you merge this, you should 1) register the main github repo with travis-ci and 2) change the "badge" on in the README.md to point to it.

The clang build currently fails on travis.  See this for details:
https://travis-ci.org/tarm/dnp3/jobs/22907016

This is against 2.0.0-M3.  The tip of 2.0.x currently does not pass ./dnp3test on travis or my computer.