Support metadata creation and retrieval.

A bug continues to exist, however.  Due to how Mime headers are
capitalized, setting a key "foobar" will cause Mime header
"X-Meta-Container-Foobar" to be set.  Since foobar != Foobar, the API
reports that "foobar" remains unset.

I'll need to find a way to fix this discrepency.  Header keys should
always be case insensitive for the purposes of key look-up.
Support setting and getting of custom metadata.

It's not yet complete, but it's functional enough for basic use.
Refactor: Custom header prefix checks in a single place
Refactor metadata map into unique type for easy reuse.
Merge pull request #97 from rackspace/cf-container-metadata

Cf container metadata