Implement OpenID store requests

[CydeWeys]

Per the Attribute Exchange spec, there is a StoreRequest that can be sent along with an OpenID request that allows passing of data to the OpenID provider. Most OpenID libraries for other languages support store requests, including DotNetOpenAuth, openid4java, Python, and ruby-openid. It'd be great if node-openid could support them, because I'm dealing with a situation where we have an existing provider that handles StoreRequests, and there's functionality we need that requires sending one.

[havard]

This should probably be part of work done on #96 (updating the current AX implementation). Takers are most welcome! :)