Expose account/container metadata facility to external API

[gholt]

With the commit of http://bazaar.launchpad.net/~hudson-openstack/swift/trunk/revision/50 we now have arbitrary metadata capabilities for accounts and containers. A restricted set of headers from the end user should allow access to user metadata, much like it already does with objects. The idea is that a user could send a X-Container-Meta-X: Y in a PUT or POST request and that header would be returned with future GET and HEAD requests. Similarly with accounts. Restrictions on count and size should be the same as with object user metadata.