Add issuer support

ROTP supports defining an "issuer" since version 1.5.0:
https://github.com/mdp/rotp#150

The issuer is the name of the platform where you are authenticating.

It's an optional param so it can be nil.

To specify a value you only have to add it as an option:

<code>acts_as_google_authenticated :method => Proc.new { |user| "#{user.nick}@issuer_domain.com" }, :issuer => 'issuer_name'</code>