Remove some built-in post types from Context filter

[frankiejarrett]

I think that some built-in post types need to be filtered out of the Context dropdown (e.g. Attachments, Navigation Menu Items, and Revisions) because no actions are ever available for them.

I realize this might be related to #49 because that would make the loading of Contexts relative to what Contexts are even available via AJAX. But until that can be implemented, perhaps we should remove these for now.

[shadyvb]

That perfectly makes sense, since those are actually ignored by the connector actions.