Implement connection monitoring

[gbakeman]

It would be useful to monitor the connection to the NUT server, and provide notifications if there's an issue.

In WinNUT, connection is monitored by sending an empty string to the server and looking for any response. If there is none, then the connection is considered broken and events are raised (see Nut_Socket.vb and UPS_Device.vb).

NUTClient raises:

• Connecting
• Connection Established
• Connection Failed
• (other events from Tcp class)

When moving to a MVVM type architecture, raising events may not be as helpful (since functions still need to return something) and instead make a caller deal with the issue up front by raising the exception. Or do both depending on the circumstance.