allow to connect to one server only

This changeset allow to disable block headers retrieval from multiple
servers.
Merge pull request #379 from wozz/clean

label editing in gtk
don't show actinos that require private key if the wallet has no seed
concealed flag for restore sequence
fix: store seed version
Merge pull request #381 from atweiden/concealed-flag

concealed flag for restore sequence
call check_seed in get_seed
Merge branch 'master' of git://github.com/spesmilo/electrum
Update default server list (disable servers with failed/expired certs)
ignore gui/icons_rc.py
replace BaseException with Exception
don't use bare except
Merge pull request #385 from WyseNynja/ignore_gui_icons

ignore gui/icons_rc.py
Merge pull request #386 from WyseNynja/dont_use_base_exception

replace BaseException with Exception
wallet.py: use get_seed in update_password
Merge pull request #374 from nolith/master

allow to connect to one server only
get_private_key: call get_seed first in order to check password
Switch icon based on OS, OS X requires dark icon
Merge remote-tracking branch 'upstream/master' into clean