Monadic extension for Task, 

TaskExtension provides monadic extensions to Microsoft's [Task](http://msdn.microsoft.com/en-us/library/dd321424(v=vs.110).aspx) class.  Note that constructing and comprehending Tasks is referentially transparent, only calling "Start" will kick off the computation in another thread on the environment's thread pool.   

EitherTTask<A> is analogous to an EitherT<Task,A> if .NET allowed HKT, in other words a way to comprehend over both the Either and the Task in an easy to use way.   

Sequence is self explanatory, though we may want to move it to List?