Adding async http string extension methods

I often use the ``PostJsonToUrl`` and similar string extension methods, but a lot of the time, I either don't care about the results, or want to receive them async style.  

I couldn't find the best way to do that with servicestack, so I just created more extension methods that instead return ``Task<string>``.

Is this something you're interested in adding?

I don't know how to do this PCL style, so not sure how that part would work.

Anyway just throwin this out there for thoughts (implementation of async extensions could prolly be done better), let me know what you think.