The Big AG Refactor, WIP.

* The AG now keeps track of all path metadata.  Previously, drivers had
done this.
* The AG keeps path metadata consistent automatically.  Previously,
drivers had to do this.  This fixes issues #73 and #74.
* The AG will match "query_type" file/directory tag attributes to the
appropriate driver to load and use to access information.  The AG will
ship with all drivers as a whole unit, and only download the spec file
from the MS.
* Drivers are kept as separate from the AG as possible.  The AG
implements a stable driver API, and drivers no longer interact with
Syndicate-specific data strutures (like manifests).
* Revamped driver API.  Handle manifests separately from blocks, and
have the AG take care of signing, verifying, etc. and filling in the
appropriate information.
* The AG returnes hashed blocks.  This fixes issue #62.

More to come: bringing all the drivers up to speed, more testing, and
updates to syntool.py to support pushing just the spec file.  Also,
under AG/tools, we'll put utilities for generating specfiles from more
easily-managed data structures (because I doubt curators will want to
write raw XML, in specifically the way the AG expects).