Add a speakers page in the admin section

This is based on #270 as it shares a lot of the styling changes, so that should be merged first.

Add an additional page to show just speakers (i.e. users who have submitted sessions).
> The implementation started out loading data from the database using Simple.Data, and so was a repository

That doesn't necessarily follow. See this example loading data from RavenDB: http://lostechies.com/jimmybogard/2012/10/08/favor-query-objects-over-repositories/

> From the interface point of view, I think it should be consistent with the others (i.e. either they are are query objects or all repositories).

That is a fair point. Perhaps leave it for now and we can take a look at refactoring the repositories to query objects another time.