add minimal implementation of search lucene

This PR adds a minimal "Full Text Search" application derived from the work in https://github.com/owncloud/core/pull/849. Please try it out and provide feedback. If you give me your :+1: It might even be shipped with OC5!

@andrewsbrown @jancborchardt @karlitschek @tanghus @DeepDiver1975 @LukasReschke @torlasz please test

@icewind1991 please have a look at ajax/status.php where I try to find out which files need indexing. I don't know if I understood the Storage/Cache/Filesystem relationships and ids correctly.