Constructor for low privilege (no clientSecret) api access.

For https://github.com/auth0/auth0.net/issues/21

Also enabled XML documentation on build, should be part of Nuget package.
Let's use invalid operation exception http://msdn.microsoft.com/en-us/library/system.invalidoperationexception(v=vs.110).aspx. I think it better explains what is going on.

I would expect a security exception if I actually reach the resource but can't act on it. This is just that the operation is not valid for the current object's state.