Revert "Reference to Microsoft.Bcl.Build for PCL-Tests"

This reverts commit 8bbf2bf6940d8a7b1d624bad6671ee18daa761be.
Remove Reference to BCL, abstract HttpClient

I'll let the experts figure out Microsoft.Bcl.Build and the ensuing
warnings. :thumbsup:

Wrapped ProductHeaderValue class, hope this is right. Made some changes
to ensure the project uses the new, wrapped, ProductHeaderValue.