Explicit type for .ReadAsAsync() and .ReadAsMultipartAsync()

Explicit typing for 2 methods of HttpContent as the new version of
referenced dll has multiple overrides for HttpContent.ReadAsAsync() and
HttpContent.ReadAsMultipartAsync().
Merge pull request #15 from Thorium/master

Explicit type for .ReadAsAsync() and .ReadAsMultipartAsync()
Fix build script.
Use an F# only web project in the sample.
Add original documentation introduction and HttpApplication type alias.
Apply the request to the response when calling respond.
Update build.proj to use stable FAKE version.
Use an Option type for content.