Use Travis for continuous integration

[fschopp]

Sign up for Travis and add .travis.yml file for configuration.

Ideally, the following items are addressed:

• Grid Engine should be installed so that the integration tests in project cloudkeeper-drmaa-executor can run.
• The project documentation generated by mvn site (including the JavaDoc) should be made available on the web (ideally, for recent releases as well as for the current master branch).