Search lucene refactoring

This is https://github.com/owncloud/apps/pull/1624 moved to this Repo. It does not change the database.xml but changes the way backgroundjobs are used to index new files and update changed files.

Also starts adding unit tests and initial README.md Will enable travis testing and add more tests in future PR.

@DeepDiver1975 @karlitschek @kabum see commit messages for detailed changes.
Maybe split the class in something like:

- `FileStatus`: single file status, more or less the non-static methods from this class
- `Status`: overall status, the static methods of this class, only made non-static