Added Gradle files for AAR publishing

I replaced the Maven build system with the new Gradle build system, which is needed for AAR publishing, based on Chris Banes' article: http://chris.banes.me/blog/2013/08/27/pushing-aars-to-maven-central/.

To get the AAR publishing working you need to have a Sonatype Nexus login. The login credentials should be placed in a `gradle.properties` located in your local gradle directory. On Unix systems this directory will be located under `$HOME/.gradle`. The `gradle.properties` file should look like this:
```
nexusUsername=xxx
nexusPassword=yyy
```

In this step, I also removed all Maven and Eclipse related files.