redefine Message.MaxRetransmit

the default value of 0 means using ICoapConfig.MaxRetransmit
make ICoapConfig observable
threading
clean up
use TPL as default executor for .NET 4.0
remove project files for vs2005. won't be able to be compiled in vs05 for a heavy use of lamdas
apply SocketAsyncEventArgs instead of APM for .NET 4
version 0.18
dispose resourses in UDPChannel
avoid calling SocketAsyncEventArgs.SetBuffer whenever possible