Change votingapi tag

Also #475612: Clean up the voting results - not just the votes. Also may affect #324051: List x number of top voted.

See also #772626: Namespace votes. I'd like to see that patch extended with a setting and an update to convert existing votes.

I discussed this with people at our Boston Meet up last night, which included Moshe Weitzman. The consensus was that, while unusual, it is not out of the realm of possibility (and, indeed, has happened) that two or more voting modules may exist in a single site. Further, Moshe suggested that not having a specific P1 tag is using VotingApi incorrectly.

So, I'm soliciting suggestions on how to upgrade the module with the least possible impact on sites that may already be in this situation.

I have now added a tag constant to the module and (I believe) changed all the criteria to use it. So at least new users can easily switch.

With the latest commit, I now consider the #1 issue and need feedback, particularly if you use more than one voting/rating module on yours site.