handler/PUT: Handle signature verification through a custom reader. (#2066)

Change brings in a new signVerifyReader which provides a io.Reader
compatible reader, additionally implements Verify() function.

Verify() function validates the signature present in the incoming
request. This approach is choosen to avoid complexities involved
in using io.Pipe().

Thanks to Krishna for his inputs on this.

Fixes #2058
Fixes #2054
Fixes #2087

web-handlers.go

@@ -356,6 +356,7 @@ func (web *webAPIHandlers) Upload(w http.ResponseWriter, r *http.Request) {
 	vars := mux.Vars(r)
 	bucket := vars["bucket"]
 	object := vars["object"]
+	// FIXME: Allow file upload handler to set content-type, content-encoding.
 	if _, err := web.ObjectAPI.PutObject(bucket, object, -1, r.Body, nil); err != nil {
 		writeWebErrorResponse(w, err)
 	}