Add Average Cycle Time and Throughput Stats

Hi Kevin - 

I added these stats, which required a db schema change. Take a look and let me know what you think.

Changes:
* Added stats to welcome erb (Avg Cycle Time + Weekly Throughput)
* Added board_id to the cards table. This includes updating the API call for cards to get the idBoard field/value back from Trello. Also the accompanying db migration script.
* Added supporting methods to the board.rb. (I admit to a little repetition in there.)

Cheers
Ben