Support for gradle files

This pull requests add support for Gradle files as discussed in #1766.
It's using [alkemist/gradle.tmbundle](https://github.com/alkemist/gradle.tmbundle) for syntax highlighting.

I'm going to open an issue at [alkemist/gradle.tmbundle](https://github.com/alkemist/gradle.tmbundle) to clarify the license situation.