Update build system to Maven

Still may need some improvements, but builds without errors and allows the usage of the same library versions added to the project as dependencies.

I've also added a small change when the backup file is not found in the datastore: instead of a log to stderr, a new IOException is thrown, so the task queue can retry. In practice, this case only happens if the backup itself isn't showing up in the datastore, but the _ah_builting_python_bundle version may be creating the meta information.

Also, this time, the changes are far less than the previous pull request, since the last commit to the opensource project removed some unused code relying on the MapReduce library. Basically, I'm removing the dependencies and replacing them to the equivalent pom.xml at the root folder. I've also updated some versions in the process, that may fix some bugs.