Virmani/gradle upgrade

Hey guys,

I just started looking into this project. This pull request seems big but it contains the following 2 simple changes: 
- migrated the project to start using gradle. I have used the Android Studio 0.8.1's import tool for this. 
- updated the .gitignore file to contain all the paths that need to be ignored for a gradle based Android Studio project

I am not sure if you guys are ready for this right now or not but IntelliJ works best for me. Moreover, android development is moving towards gradle and android studio. Let me know if this seems ok.