Lazy initialization of RaygunClient

Avoids the client calling setUser() in its constructor,
and hence forcing a session_start() to track the user.
This causes all kinds of nasty side effects because
it happens before Session::start() is called, e.g.
when using "Session.cookie_secure".
Merge pull request #10 from silverstripe-iterators/pulls/lazy-init

Lazy initialization of RaygunClient