Gradle support, Workspace cleanup, Nickname persisting

Hi, nice game :-)

I've made few changes

  - Removed all Eclipse and local build files (gen,bin,.settings,.classpath, etc... see `.gitignore`)
  - Added Gradle support (which is now in beta, but f-droid team is already handling it)
  - Persisting nickname (bug which made me to do this pull request, not forcing user to type in nickname everytime)