Gradle support

We have added Gradle support, following Google guidelines. We have also simplified the procedure, updated the setup instructions, and tried to address the Maven problem (#654).

It should be mentioned that in order to be able to build with Gradle the library must support Gradle as well, so we have changed where the submodule points to to our fork until owncloud/android-library#38 gets merged, as indicated on #625.

Edit: ~~we're looking into the Travis configuration, we forgot to address it.~~ We have fixed the Travis configuration, also including Gradle build evaluation (see #654 for a discussion about Maven). @davivel we have seen you've been trying to fix the Travis configuration earlier today; probably you want to take a look at the one we suggest.