Travis config

Here's a basic configuration which builds the library and examples on Travis, using the Terry Guo gnuarm PPA toolchain. You can see the builds happening in my fork:

https://travis-ci.org/mikepurvis/stm32plus

If you go to https://travis-ci.org, sign in, and click the switch to enable builds for your stm32plus repo, it should kick off the first build once the .travis.yml file is merged in.