add Gradle files to Android.gitignore

Information about using Gradle to build Android projects:
http://tools.android.com/tech-docs/new-build-system

Seems like Gradle documentation lacks of this description
Folder build/ uses as output of build process. 
As you can see here http://www.gradle.org/docs/current/userguide/userguide_single.html#N103D7 "clean" task removes content of this folder as it always generates.

.gradle folder creates inside a project (if it uses Gradle wrapper). It's purpose is the same as for example .m2 folder for Maven configuration.
Gradle wrapper is separate Gradle build which does not requires setup on computer and always puts into project root directory. It simplifies project setup on different environment.