Add travis continuous integration support

This commit adds support for continuous integration on clang and GCC using http://travis-ci.org/ (a free service).

In order to enable this, someone with repository admin permissions (@ridiculousfish, not sure about anyone else) needs to follow steps 1 and 2 from [the getting started guide](http://docs.travis-ci.com/user/getting-started/#Step-one%3A-Sign-in); then this commit (which implements step 3) can be merged.

The benefit is that all commits and pull requests are built to make sure they compile and that the unit tests run.