Improved async.md Cancellation Token example.

[isaacabraham]

Improve Cancellation Token sample.

Summary

Having reviewed the Cancellation Token source example, I'm not sure it is currently a particularly good example:

1. It doesn't do anything F# asynchronous inside it - it actually calls the "original" Async web client upload async, not the F# async version or even the C# Async / Await version. This is confusing, as it doesn't block and can't be let bound.
2. The workflow returned immediately - there was no opportunity to see cancellation working in action.

I've replaced it with a simpler example that simpler prints to the console every 1 second. The cancellation token can be called, which will stop the workflow. This works very well from within a script.

[cartermp]

Thanks @isaacabraham this is much better.

[Thraka]

@isaacabraham Hi Isaac! Looks like something came through and caused a merge conflict. You'll need to fix this before we can merge. Thanks.

[mairaw]

@isaacabraham @cartermp can one of you take a look at the merge conflicts and see which version of the code (or a mix of the two) you prefer to keep?

[isaacabraham]

Oops - will do.

[isaacabraham]

@mairaw fixed. @cartermp it should be ok now - it uses the second (simplified) version.