Monadic extension for Task,

[vmarquez]

TaskExtension provides monadic extensions to Microsoft's Task 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?