Fix OpenID id / schema regular expressions

* Use regular expressions for OpenIDs consistently.
 * Relax OpenID regular expressions (allow https, allow missing trailing
slash)
 * Fix Yahoo email attribute exchange (addendum to
cfe046e651c441bf0be49fac404c2e81d75ddeee).
Use generator expressions instead of map

This saves some code and some time :)
Make User.find_by_email case insensitive

Addendum to eb595d326b6141f96fe19ea4c7e5a09de88d80c2, see discussion in
#214.

While at it, remove unneeded exception.