Update itbit/coindesk exchange rate api

itbit/coindesk exchange rate requires separate API calls per currency
Moved the calls to when the exchange rate plugin updates and when the
user modifies a selection instead of every time the program needs to
query the current exchange rate

This fixes an issue with slow response time in the send tab with the
exchange rate plugin turned on and set to coindesk or itbit
Merge pull request #820 from wozz/exch_update

Update itbit/coindesk exchange rate api