Add require statement as recommended

Without the explicit require statement in the strategy (as recommended in the
OmniAuth Oauth2 README) it is necessary to either explicitly add the
`omniauth-oauth2` gem in the Gemfile or otherwise require it.
Remove dependency on undefined model 'Account'

There is code that references `Account`, but that model is not defined.  This
code change preserves the original functionality if that model is defined, but
allows broader use of the Gem with applications that do not have an `Account`.
Merge branch 'hotfix/remove-dependency-on-account' into devel