Travis CI

Use [Travis CI](https://travis-ci.org/) for builds.

My Jenkins server is not very stable, and Travis CI has awesome Github integration. I propose to use Travis for builds going forward. It even builds any incoming Pull Requests, and marks the Pull Request with whether the pass or not.

Because Travis doesn't have an Android environment ready, we have to use a simple Java environment for now, and download the Android SDK and stuff on each build. There is a hard time limit on each build step too, so not all platforms can be downloaded. For this, I have streamlined the tests to only require `android-16`.

If/when you accept this pull request, please do both repos (`maven-android-plugin` and `maven-android-plugin-samples`) at the same time. They are both updated to only require `android-16` and the first build of either would fail if they are not both updated then :)