Add scope feature for Voter method 'unvote_for':
Update Voter method 'unvote_for':
- add optional parameters 'args={}'
- pass to method votable.unvote additional parameter ':vote_scope => args[:vote_scope]'

This change allows use similar form for voting and unvoting by voter with scope. Otherwise unvoting with scope can by done by votable only.
Merge pull request #66 from Kondratenko-Valeriy/master

Add scope feature for Voter method 'unvote_for'