Updated pom

I updated the POM so maven build is working again. It was quiet tricky because the Android Studio/gradle project structure is very different from the classical Android Eclipse/maven structure.

I also added CONTRIBUTING.md which includes usage instructions for the maven build. It can be added to the wiki, or leaved as is, as you want.

With this update, building from maven command line is ok, importing to eclipse also works, but building with eclipse not, since the m2e and m2e-android plugins are not so flexible to use the gradle project structure.

Running the tests from maven is also problematic, since the tests are simple unit test-type, but these cannot be runned as is because android.jar contains stubs. The test should be instrumentation test or use Robolectric. IDK how gradle/AS handle this though, so i did not changed the tests.
However, it would help a little if the name of the tests folder would be the traditional `test` and not `androidTest`.