Android plurals

Here is a clean and rebased pull request in order to add support for Android plurals, as I said here https://github.com/translate/translate/pull/23#issuecomment-31671327.

I actually delegated the Unicode CLDR plural list maintenance to Babel (added as a new dependency), in order to get a cleaner and more maintainable code.

I also added a couple of simple tests.

Bye,
Luca