Misc tweaks and cleanups
Tweaked FeedUri.ToStringRfc()
DesktopIntegration: Fixed handling of missing icon
Added Requirements constructors
Updated NanoByte.Common
Use object output instead of string output for "0install list" and "0install list-feeds"
Changed "0install store list" to list stores instead of implementations

Added "0install store list-implementations" as a replacement
Added "0install search"
Introduced IPackageManager, NativeImplementation and NativeRetrievalMethod
Detect native Windows software packages (incomplete)