Added System.Progress<T>
New SpinWait methods for ThreadingHelper.

Renamed SpinWait to SpinWaitUntil
Added SpinWaitWhile and SpinWaitWhileNull
SpinWaitWhileNull is now generic
Fixed timeout implementation
Added ManualResetEventSlim
Added TaskStatus and TaskCreationOption
Added Interleave, InterleaveMany and ZipMany exts
Renamed Extend to Append

This name is more popular
Append improvements and more renaming
Added Prepend and rearranged code
Added ExceptWhere
Added IsEmpty extension method