Provide metadata functions for tags and trash support. Restructure backend to work with blocks and hashmaps.

List of major API updates (tested and ready changes are in the docs):
* List object meta keys in container headers.
* Filter object listings based on their meta keys.
* Enforce metadata replacement on POST/COPY/MOVE.
* Add Content-Range to range replies.
* Return ETag in If-None-Match GET replies.
* Handle multi-range GET replies as outlined in RFC2616.
* Add Content-Disposition to the allowable object headers.
* Implement block per block GET and PUT.
* Better range parsing (and multiple ranges).

The new backend is version-ready, but the API does not yet implement versions. The frontend exposes hashmaps through GET - this may change.

Fixes #559
Fixes #561
Refs #563