Initial implementation for Firefox OS

Implemented File System API on top of indexedDB, based on [this shim](https://github.com/ebidel/idb.filesystem.js).

I wrote a jasmine test app to help me implement this, you can find it [here](https://github.com/rodms10/FileTest). If needed, I can clean the tests and try to incorporate them on whatever test framework it fits into.

@zalun, can you take a look?