Provide Task<IRestResponse<T>> methods

[haacked]

Also provided simpler extension methods when you don't care about the
response info and just want the data.

Fixes #423

[half-ogre]

Seems reasonable. Having to put Task in every name sucks. Maybe {Operation}ResultAsync instead? (Yeah, I don't like it much either.)

[haacked]

Having to put Task in every name sucks

Yeah. It follows the standard guidelines when you already have the other obvious name GetFoo and GetFooAsync taken by existing methods. I'm going to follow the standard practice even if it's ugly so people know what to look for and expect. Thanks for the review!

[half-ogre]

It follows the standard guidelines when you already have the other obvious name GetFoo and GetFooAsync taken by existing methods.

Good to know; I wasn't aware of that. Where does these standard guidelines live? I should probably read them.

[haacked]

Here you go.

http://msdn.microsoft.com/en-us/library/hh873175.aspx