Refresh gradle configurations

A few things, all to refresh the current gradle configuration to support the latest versions of Android Studio

* Tweaked `.gitignore` to better handle IntelliJ/Android Studio files
* Update gradle plugin version to latest `1.0.0-rc4`
* Add gradle wrapper for easy project import
* Updates plugin names to new versions (previous versions were deprecated and will break soon)
* Update build and compile SDK versions to latest
* Remove unnecessary gradle plugin version and repositories declarations, as these are set via the top-level build.gradle file
* Update target SDK version to latest 21
* Specify min and target SDK versions that match the manifests
* Update `README.md` to use syntax highlighting in its code samples

Tested the sample and everything still works as normal, and everything should still continue to work fine in Eclipse as well :)