Add travis continuous integration support + bugfixes

This PR adds a .travis.yml file which provides setup instructions for travis CI (https://travis-ci.org/). The idea is that each commit or PR will trigger a rebuild on travis which reports whether all unittests are still passing. 

There is a minor issue with this however. Travis does not seem to support multiprocessing. I circumvented that by adding a test whether multiprocessing is available in the appropriate tests.

Finally, there were some obvious regressions in the tests and some examples which I also fixed.

Once this is merged, someone from pymc-devs will have to activate travis for this repo.