Add history fiat tx value for exchange plugin

Added coindesk BPI rate calculation for transaction values in history.

Coindesk currently only provides this functionality for USD, so it is
only available when the Coindesk and USD combination has been
selected.
update for new transactions with exchange plugin

new transactions aren't already loaded to the wallet when the update is
run. this catches the exception and gets the transaction value
Merge pull request #596 from wozz/exchangerate-history

Add history fiat tx value for exchange plugin