Gradle building system

This pull request adds support for the Gradle building system. The folders' layout has been kept which allow full compatibility with Apache Ant as inidicated in https://github.com/owncloud/android/issues/625.

To use Android Studio and not compromising the folders' structure is necessary to import the project by selecting the existing **build.gradle** file in the root directory of the project and not the root directory itself.

The pull request also affects some files that contained non UTF-8 characters in the Java docs. These files were breaking the compilation process. We believe that this change is mandatory since it is considered a bad practice also by the Apache Ant since they raise warnings. The modification just removed those characters since we couldn't agree which markup to use to highlight such docs sections and we leave this decision to you.

Future works will extend the new building system to the Android application main repository.
Just for the record: this was rolled back because resulted in more problems for new developers get the project working in Studio than the original option.