Making UrlHelper's methods virtual

/cc: @rynowak @yishaigalatzer 
According to Yishai, we should make TypeHelper public (its currently internal)...I considered this possiblity to avoid creating this new virtual method, but felt its probably not easily discoverable...but on second thought, if a user is writing a custom url helper he probably has more knowledge of how things work should be able to figure this out...thoughts on removing these new virtual methods?