Generate gradle wrapper to work with versions

Only build.gradle is added by me. Other files are created when you run the 
command gradle wrapper.

Gradle version 1.12 is required for our gradle. But by default
most of us has only 1.4 . So the tasks will fail with compile
errors in these cases. Gradle provides a wrapper where it manages
the versions for us. Add task gradle wrapper and
it creates some files. These files can be checked into the
version control and people can run this command instead of gradle.