Updates build tools to latest version & adds entry to gitignore

This PR updates many things:

 * Makes use of latest gradle for the wrapper (2.1)
 * Updates android plugin definition to `com.android.library`
 * Uses android gradle plugin 0.13.0
 * Uses `compileSdkVersion` 20 and `buildToolsVersion` 20.0.0
 * Adds build/ folders to gitignore