Update .gitignore

Hi!

I'm using this library for one of my projects and wanted to check out the source, tried to import this into Android Studio and I noticed that gitignore wasn't ignoring the build files,so I thought I'd send the fix as a pull request.

Also, your build.gradle file contains the config for uploading to maven, which isn't valid for anyone else who wants to clone your code. I'm unfortunately a total beginner to groovy so I have no idea on how to fix this, but perhaps you do!

Also, kudos for the great library!

-Nikhil