Updated documentation for container and account methods in IObjectStorageProvider

- Fix confusing handling of non-metadata header updates (fixes #107)
- Fix confusing results returned from DeleteContainer (fixes #108)
- Fix inconsistent handling of metadata keys (fixes #110)
- Simplify DeleteContainerMetadata to take IEnumerable<string> instead of Dictionary<string, string> (where values were always ignored)
Update documentation for object methods in IObjectStorageProvider