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 switch-case ---> Review area.js for an example