Add Gradle wrapper
Remove Maven build files
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.+
- Remove Maven commands since project is now using Gradle
- Remove unnecessary default settings
Update Gradle build tools and wrapper
Add build status indicator to README.md