Crashfix: 'From' jid w/ non-ASCII char sending cmd

Fixes a crash occurring when a remote jid with a
non-ASCII character issued a XEP-0050 Ad-Hoc
command.

Specifically, jid=str(...) was changed to
jid=unicode(...) in commands.py. No side effects
observed.
Merge pull request #10 from ivucica/FixCommandsForUnicodeJids

Crashfix: 'From' jid with a non-ASCII char sending a command