[rackspace] Add support for container metadata

- client.createContainer re-worked to take options instead of container
- added client.updateContainerMetadata which does the appropriate PUT request
[rackspace] Adding support for file metadata

- reworked container metadata to use private _updateContainerMetadata function
- added updateFileMetadata function
[rackspace] removing File.addMetadata in lieu of File.updateMetadata

fixes #144
[docs] Updating documentation for metadata changes
[rackspace] adding metadata methods on container model
Merge branch 'storageMetadata'
[minor] updating changelog