Added Keep Alive Feature for .NET client and tests

- Added the Keep Alive class for storing timeout values
- Added HeartbeatMonitor to fire timeouts and timeout warnings
- Added Timeout and Timeout Warning events to Connection
- Added unit and functional tests
Added the HeartbeatMonitor and KeepAliveData classes to the Silverlight project
- Removed the timer code from the WinRT project

- Fixed a syntax error in WinRT.csproj
Made changes in 'using' statements as per code review feedback
Added an using statement for Timer in HeartbeatMonitor
Changes made as per code review feedback
Added the SupportsKeepAlive property to the various transports
Added KeepAliveData and HeartBeatMonitor files to the WP8 project