No longer require restart for toggle exchange rate

Fixes issue #643 and allows fiat button to work without restart

No logic is required because Send tab gets removed and added for every
toggle.
relocate export_history code
handle coinbase transactions explicitly
Exchange rate update

use exchange rate value instead of string to calculate balance
Merge pull request #665 from wozz/exchange-value

Exchange rate update
installwizard: multisig wallets
fix #655
abstract and improve seed and key methods
more robust get_seed_text
fix: do not lower xpub
Merge pull request #662 from wozz/issue-643

No longer require restart for toggle exchange rate
lower default fee. fixes #403
simplify getseed: do not return hex
switch default display to mBTC
no accounts in multisig wallets
Add block explorer setting for "view online"
Merge pull request #667 from wozz/blockexplor

Add block explorer setting for "view online"
get_master_public_keys
Merge branch 'master' of git://github.com/spesmilo/electrum
dark/light icon selector. see #666