Add a simple travis configuration

This configures [Travis](https://travis-ci.org/) to build and test all pull requests and pushes to OpenTSDB. You'll need to [enable](https://travis-ci.org/profile/OpenTSDB) the main OpenTSDB repo on Travis for it to work. You can see what it looks like [here](https://travis-ci.org/luuse/opentsdb/jobs/29037241).

It's configured to run on Oracle JDK 7 and OpenJDK 6 (Sun JDK 6) is not available on Travis. It both compiles and executes the tests using maven because that's a bit easier to integrate with other tools for java :).

The tests currently fail. I'm not sure if this is expected or not.

Do we want a badge or link in the README to this?