Migrate seadroid to gradle / Android Studio

This pull request migrates seadroid to gradle. All dependencies are downloaded from maven central or are provided in the libraries folder.

After pulling seadroid can be build with `./gradlew assembleDebug`. Seadroid gets build and shows possible errors and lint messages. I also changed the travis build to use the android type, but I haven't test it. 

Android SDK versions and build tools can be configured in the root `build.gradle` which is applied for the application as well as the libraries. As a bonus the version code is the number of commits and the version name is the last tag + number of commits since then + hash. When releasing, a new tag should be created, then only the tag name is shown for the version.

Files for maven build were removed because they won't work anymore.

To open seadroid in Android Studio / IntelliJ select import -> from gradle -> next -> finish. The project will be imported and after a short while one can start programming.

When releasing, uncomment the `signingConfigs` lines and build the app with `./gradlew assembleRelease`. An apk should be in `build/output` afterwards.

Fixes #176.