Added .editoricon.png icon used by "Solution Icon" extension

- see https://github.com/ashmind/SolutionIcon
AsyncLINQ: added Prefetch, Batch and Window operators

- Prefetch always start the next MoveNext() on the inner sequence before returning a value
- Batch is the inverse of SelectMany and outputs fixed size arrays of items
- Window is equivalent to batch, but looks at the latency of each item and produces batches of items that arrived at the same time
- need more unit tests!