menu tokens, freetagging help, error when no term is selected at all

attached patch does three things:

1) it does a quick query (only when necessary) to try to get some data that might not be available with complex data types, though it requires the consuming module's weight to be low. The thing about weights should probably be documented in the API.txt

2) it provides menu tokens and a function to help with that

3) it prevents an error by only calling taxonomy_get_vocabulary if we have a vid

I should break it up, but that would be a pain.