Added Omni.Foundation.Network.ReachabilityMonitor.
Added OmniObject.

I also added the NSDictionary.GetIndentedDescription() method in
XamMacExtensions.
Added the SafeGetHandle() method.

This method is a replacement for NSObject.Handle. It returns
IntPtr.Zero if the receiver is null, instead of throwing a
NullReferenceException.
Removed a spurious Solution Folder.