Changed market exchange rate data source to Bitstamp.

Retained mtgox code, but referencing exchange rate from Bitstamp API
to better reflect market rates (goxbux price has significant deviations
from other major exchanges).
Added changes from add1ct3dd for accepted shares display

from fork add1ct3dd/PiMiner 4e50d35aabe43f5dd13c5a822c95a147b5ef7df3

Update PiMinerInfo.py
Format floating point for avg hashrate to 5 decimal places

For screen 1 avg hashrate that fits within display.
Catch possible ValueError in loading price data

Catch possible ValueError exception and prevent the program
from crashing if the market price data could not be obtained
from the external data source.
Added trailing slash to bitstamp API url as previous was returning access denied from bitstamp.net.
Merge pull request #3 from denmojo/master

Change market price data source.