Add Travis CI config to run tests on each commit and each pull request.

This CL adds a [Travis CI](https://travis-ci.org/) config to run tests on every commit and send an email, if they failed. It also adds a build status banner, like below:

[![Build Status](https://travis-ci.org/krasin/cadquery.svg)](https://travis-ci.org/krasin/cadquery)

The only difference is that the banner above points to my fork, and the banner proposed by this CL points to the not-yet-existing profile for dcowden.

To enable Travis CI on github.com/dcowden/cadquery, the following quick setup is needed:

1. Go to https://travis-ci.org/ and click "Sign in with GitHub" button in the upper right corner.
2. Grant the requested permissions (it will not ask for code access, just commit hooks/list emails/access project activity)
3. Go to https://travis-ci.org/profile and enable builds on dcowden/cadquery repository
4. Merge this CL and enjoy the reduced risk to break the test and not to get noticed.

Please, let me know, if this CL needs more clarification. I would be happy to address any concerns.