Adds support for gradle

Hi

Thanks for an excellent lib. It's hard to find but it's exactly what we needed :)

The lint tool will report an error if the project is a gradle project. Instead of running ```lint``` the script should run ```gradlew :lint```
I've added the support for gradle to the ArcanistAndroidLinter class. It will check if the project has ```build.gradle``` and run ```gradlew``` or ```gradle``` depending on whats available.

Thanks,
Tom