Support for Windows Phone 7.1

Although TypeProviders don't work for Windows Phone 7.1, the other components of FSharp.Data are still very useful:
* CSV Parser and Reader
* JSON Parser and Reader
* HTTP utilities

(the reason TypeProviders don't work is that the latest official version of FSharp.Core is for F# 2.0, and although it's possible to compile F# 3.0 from source, the code will fail to package when deploying to the emulator or a device because strong name validation will fail due to not being signed by the real Microsoft snk)

There are no specific tests for Windows Phone added, as having tests run in that runtime is complicated, but I have thoroughly manually tested this and I actually have two published apps running using the code of this branch

There are also a couple of fixes for the HTTP runtime of the portable version when running under WP8 and Win8 (reflection related) included in this pull request