Add Android .gitignore
Update targetSdk and versionName
Add project.properties files

These files are required for Eclipse to pick up the correct target sdk
level after importing the projects.
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