Implemented stats page

This pull request adds the ability to track statistics that might be interesting.

First off there is a new folder called lib added that implements the backend for saving the stats. Right now the default engine is a file-backed one but that can be swapped in for something like leveldb if omniwallet ever moves away from flat files. 

The API folder has a new file called stats.py that has one method that generates stats based on the persisted wallets that live on this omniwallet instance. This file should be run one cron once an hour or so.

The broadcast method has been changed to also increment the corresponding stat on success.

All this together is being used by the new stats page that has been added and displays these stats by retrieving them via a JSON request.