Update Readme to specify hard requirement on Ruby 2.0-p247
Change version to 0.4.0. Rely on easy_auth 0.4.0
Use const_get and rescue from NameError instead of eval()

Module.const_get can't parse nested classes. We can try to load the provider
class ::Identities::Oauth2::Provider and rescue from the error if it doesn't
exist.
Use PG Array Overlap to find the identity for the given UID

Now that EasyAuth 0.4 uses arrays on the UID column, we need to find by overlapping
the retrieved UID from the client on the table.