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
Made changes to the KeepAlive feature as per code review feedback

- Added a SuccessiveTimeoutTest to test the case when the connection times out successively
- Added constructors to the KeepAliveData class
#741
Made changes to the KeepAlive Feature as per code review feedback

- Added an Interlocked Exchange variable to Beat in HeartBeatMonitor to prevent race conditions
- Added a null-check before starting the HeartBeatMonitor in Connection to prevent null exceptions

#741
Fixed a test in TransportFacts by mocking the UpdateLastKeepAlive method in connection.