Support taxonomy (not content_taxonomy) fields

Seems to work okay. Re: my thoughts at #644444: Generic callback functions, the getter/setter callbacks work with arrays of term ids, import/export functions try to be a bit cleverer.

The export callback returns an array of term objects. I think still to do is to add to those objects a full vocabulary object (output of taxonomy_vocabulary_load()) in place of the vocabulary id. That way people exporting to non-Drupal systems can better piece together the relationships between terms with just the exported nodes to work with. An alternative/complementary approach would be to make vocabularies exportable entities.

The import callback will try to work with pretty much anything: a single integer/string, an array of integer/strings, and array of arrays, an array of objects. When given a numeric value it will try to find a term with that id in the vocabularies that apply to the current content type. When given a non-numeric string or array thereof, it will try to find the corresponding term by first trying taxonomy_get_term_by_name(), then taxonomy_get_synonym_root() if nothing's found, again limiting to appropriate vocabularies for that node type.