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
Merge pull request #134 from sharwell/fix-126

Cleanup IObjectStorageMetadataProcessor