Bitcoin Rate Script Release

Added a bitcoinprice.js file that queries the Coin Desk API for the current going rate for a bitcoin. The user specifies which currency they want the rate to be displayed by specifying "USD" for US Dollar, "GBP" for British Pound Sterling, and "EUR" for Euro. 

Example Command:
"hubot show bitcoin in USD"
should be "an input", although I don't think this part is necessary. Can probably just go with desired currency type, seeing as how that's the only acceptable input