This PR moves the `vfs.py` module, which is only used by plugins, to
avoid polluting the main beets namespace. Also exposes the `vfs` and
`art` module from beets with a deprecation warning.
* Modify the "--musicbrainzid" argument to the importer so multiple IDs can be
specified by the user instead of a single one.
* Revise autotag.match.tag_album and autotag.match.tag_item signature to expect
a list of IDs (search_ids) instead of a single one (search_id), and add logic
for handling and returning multiple matches for those IDs.
* Update calls to those functions in other parts of the code.
Include import of __future__ features division, absolute_imports and
print_function everywhere. Don't add unicode_literals yet for it is
harder to convert.
Goal is smoothing the transition to python 3.
Instead of parsing the template at each call to destination(), it's now possible
to parse them *once*, a priori, and re-use the resulting template object. This
is analogous to the re module's compiled expressions.
