I added a client certificate parameter to AsyncRequestDetailed, AsyncRequest, RequestDetailed and Request
included Ovatsus' comments 

need to add constant FX_NO_WEBREQUEST_CLIENTCERTIFICATES to https://github.com/fsharp/FSharp.Data/blob/master/src/DefineConstants.targets
added failwith to if fx_no_webrequest_clientcertificates
added FX_NO_WEBREQUEST_CLIENTCERTIFICATES targetframework groups
changed "cert" to certificate and added a short example to /samples/library/http.fsx
Merge pull request #152 from remkoboschker/patch-1

I added a client certificate parameter to AsyncRequestDetailed, AsyncReq...