Introduced FeedUri class

- Replaced InvalidInterfaceIDException with UriFormatException
- Renamed FeedID properties and arguments to FeedUri
- Renamed InterfaceID properties and arguments to InterfaceUri
Always prepend file:/// when constructing FeedUris to work around a Mono bug

new Uri("/some/path").GetComponents(...) will raise a SystemException on Mono.
new Uri("file:///some/path").GetComponents(...) works.
Added support for external package managers