Expiration for signed global ids.

Adds #27.

At the moment this is for some discussion with some code.

I've taken the approach of only storing `expires_at` and converting from that when accessing `expires_in`.

I have some trouble condensing just how `expires_in`  and `expires_at` work together, and on top of that, they should work on both the instance and class level... :grin:

For instance with this set up turning off `expires_in` (by passing `nil`) at the instance level wouldn't work, because it would use the class level `expires_in`.

Basically I just want to know if I'm headed somewhere :smile:
Does that mean that the expiry will keep getting smaller if you don't restart the server? It should probably be 1.month per generation. Not a static value. 

> On Aug 22, 2014, at 8:39, Kasper Timm Hansen <notifications@github.com> wrote:
> 
> In lib/global_id/railtie.rb:
> 
> > @@ -15,6 +15,9 @@ class Railtie < Rails::Railtie # :nodoc:
> >        app.config.global_id.app ||= app.railtie_name.remove('_application').gsub!('_','-')
> >        GlobalID.app = app.config.global_id.app
> >  
> > +      app.config.global_id.expires_in ||= 1.month
> Because I resolve this to setting the expires_at attribute, this makes the passed in value a static setting. The expiration date is set to one month in the future of when this app was booted.
> 
> —
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