create gradle project and clean up ant project

Add gradle project for those who have switched or are switching to Gradle and
want to use HockeyKit. Delete R.java, because it is generated. Add a .gitignore
to ignore all build output.

Gradle wrapper checked in as recommended by
http://www.gradle.org/docs/current/userguide/gradle_wrapper.html
fix android lint issues
document gradle steps
Merge pull request #166 from FabianFrank/gradle

create gradle project and clean up ant project