Feature/use gradle build system

I've imported the project in Android Studio and converted to the Gradle build system.

I've also set up a Jenkins server at my private server to automatically build and assemble (unsigned) APKs: http://www.griffenfeld.dk:8080/job/LockscreenDisabler/lastSuccessfulBuild/

When a build fails (e.g. due to bad code being checked in), one can get a notification by subscribing to http://www.griffenfeld.dk:8080/job/LockscreenDisabler/rssFailed