Add CoinDesk BPI for exchange rate

Add CoinDesk BPI for exchange rate options. Choose between blockchain
and coindesk on the plugin settings page.
Add link in history to view the tx on blockchain

Mostly a convenience thing. webbrowser should work with Mac OS, but I
have no environment to check with.
Win and Linux both open the url fine in the default browser.
fix bug in chunk validation
Merge pull request #591 from dabura667/Blockchainlink

Add link in history to view the tx on blockchain.info
show txids in the list of transaction inputs
Merge pull request #588 from wozz/for-upstream

Add CoinDesk BPI for exchange rate
Set default exchange to Blockchain.info

set default exchange to Blockchain.info, which will force the config to
be updated if the exchange is changed.
Add tools menu function for Verify message

add separate menu option to verify message because verifying a message
does not need to be tied to a specific address
improve help messages for options
fix createrawtransaction (issue #574)
Merge pull request #592 from wozz/exchange-settings

Set default exchange to Blockchain.info
change remove tabs for verify/sign

verify message is in the drop down menus and sign is in the receive tab
drop down for each address
Merge branch 'verify-message' of github.com:wozz/electrum into wozz-verify-message
cleanup
add ok button for currencies

also update the currency list when changing exchanges.  defaults to
first in list if current selected currency is not available on the other
exchange
Merge pull request #594 from wozz/exchange-settings

add ok button for currencies
Add Coinbase exchange rate option

Add Coinbase exchange rate option in exchange rate plugin
Merge pull request #595 from wozz/coinbase-rate

Add Coinbase exchange rate option
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