Cleanup IObjectStorageMetadataProcessor

* More explicit `IObjectStorageMetadataProcessor` documentation
* Updated `CloudFilesMetadataProcessor` to use `StringComparer.OrdinalIgnoreCase` (fixes #126)
* Updated `CloudFilesMetadataProcessor` to use `string.Length` property instead of magic numbers
* Updated `CloudFilesMetadataProcessor` to use `StartsWith` instead of `Contains`