Only raise umfpack warning if cg mode is used
Monkey-patch UmfpackContext __del__

By putting the failing statement in a try: except: clause, the
exception is caught and the error message is silenced.

See
https://groups.google.com/d/msg/scikit-image/FrM5IGP6wh4/1hp-FtVZmfcJ
and
http://stackoverflow.com/questions/13977970/ignore-exceptions-printed-to-stderr-in-del/13977992?noredirect=1#comment28386412_13977992
Remove unused imports in random_walker source
Add comment and links explaining Umfpack import
Deprecate default mode 'bf' in random_walker
Add change in random_walker default mode to TODO
Merge pull request #759 from jni/supress-umf

Suppress error message during failed UmfpackContext import.