Refactored ManifestDigest
Improved exception handling

Closes #80
Updated NanoByte.Common and SharpZipLib
Introduced FeedUri class

- Replaced InvalidInterfaceIDException with UriFormatException
- Renamed FeedID properties and arguments to FeedUri
- Renamed InterfaceID properties and arguments to InterfaceUri
Refactored unit tests

Added unit tests for SelectionCandidateProvider
Added declarative way of providing local feed overrides

Closes #82
Added support for external package managers