Adds gradle-wrapper (gradle 1.6).

From the gradle docs:
> [The Gradle Wrapper (henceforth referred to as the “wrapper”) is the preferred way of starting a Gradle build.](http://www.gradle.org/docs/current/userguide/gradle_wrapper.html)

If it's a welcome change, I'd be happy to update the README to refer to `./gradlew` instead of gradle, or to suggest the following alias:

    alias gradle='./gradlew || /usr/bin/env gradle'