Make getLoginStatusUrl use /dialog/oauth

To make this a drop in replacement for the old endpoint, we also need
support from the /dialog/oauth endpoint. Specifically, the current
endpoint, when using display=none, always return the response as part of
the fragment, while the PHP SDK needs the signed_request as a query
string argument.
Merge pull request #94 from oyvindkinsey/master

Make getLoginStatusUrl use /dialog/oauth