Add CoinDesk BPI for exchange rate

Add CoinDesk BPI for exchange rate options. Choose between blockchain
and coindesk on the plugin settings page.
add timeout to daemon, update getaddressbalance
rename command: getutxoaddress
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
Merge branch 'master' of github.com:spesmilo/electrum into clean