allow to connect to one server only

This changeset allow to disable block headers retrieval from multiple
servers.
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