add tags for voting

It can be useful when you want to make voting for a complex object with some options. For example, i want make voting for a car model like honda cr-v. It has some options(reliability, economy. price and something else). This pull request provide you voting system with tags.
voter.vote_for(voteable, tag)     	   # Adds a +1 vote for a tag
voter.vote_against(voteable, tag)    # Adds a -1 vote for a tag
And we can see results by tag.
voteable.votes_for(tag)                  # show count of the +1 votes for a tag

If youy find it useful, i can update some code and readme to provide more information.