non blocking exchange rate API calls

clear combo box while waiting
Merge pull request #603 from wozz/exchange-fix-gui-hang

non blocking exchange rate API calls
change estimated_fee to include 34 bytes per output instead of hard-coded 80 (issue #520)
Merge pull request #604 from CodingAnarchy/master

change estimated_fee to include 34 bytes per output instead of hard-code...
follow-up to #604
follow up to #604 - gtk gui
Merge remote-tracking branch 'upstream/master'
Merge pull request #605 from CodingAnarchy/master

follow up to pull request #604
exchange rate bug fix
display locktime for transaction

display locktime for transactions if it is greater than 0.  Majority of
transactions will not display any value because default is 0.
BitcoinVenezeula Rates

Rates are taken from CoinDesk, modified for the true blackmarket rates,
not the "government approved" rates.
Merge pull request #607 from wozz/keyerror

exchange rate bug fix
Merge pull request #608 from wozz/locktime

display locktime for transaction
Merge pull request #609 from wozz/bitven

BitcoinVenezeula Rates
fix bug for 0 transaction wallet

0 transaction wallet would return an error when getting transaction list
Exception caused by tx_list not defined

I wasn't seeing this before, so I assume it could be a race condition if
the "load_wallet" function doesn't finish before the history tab is updated.
print statement should be debug only

comment print statement of seed
Merge pull request #614 from wozz/print-seed

print statement should be debug only
add password to sign hook
Merge branch 'master' of git://github.com/spesmilo/electrum