Added class to handle Pebble bundles.

Created a class to handle the information in Pebble bundles. Might be handy when trying to implement app/firmware upload. This implements the file structure as reported [here](http://pebbledev.org/wiki/Applications#File_Structure). The actual implementation is similar to the Python [libpebble](https://github.com/Hexxeh/libpebble/blob/master/pebble/pebble.py#L30).

I also added an example file to show case the API.