Add gradle wrapper
Update pom.xml to use gradle wrapper
Make gradlew executable on build server

- Also update Android target sdk version to 19
Apply signing plugin only for release versions

This also adds a template gradle.properties with empty signing related
variables to avoid an undefined variables error.
Fix and update Travis CI build

- Update android-sdk to r22.3
- Update support-v4 library to v19.0.+
- Install android-19 locally