Added relative path and filename and fixes to createDir

The wrapper now has support for paths like /Android/data/com.bla/files/ then one can fill in any file name and download the file.
This makes it easier to use this wrapper in my opinion, since no path like, for instance  file:///storage/emulated/0/Android/data/com.ionicframework.starter/files/copy.doc has to be provided.

Also added a deferred object and promise return at the createDir function. According to the documentation the function should return a promise, but it did not. Now it does.