[OSS] Improve MeetupBundle Isolation

[rdohms]

In order to OSS all these bundles so other groups can use them, we need to improve their isolation, for the MeetupBundle this means

• Make twig functions in the style of {{ next_event(templatename) }}
• Make group_urlname configurable #45
• Make API calls fail better #46
• Twig Function: next_event
• Twig Function: past_events
• Twig Function: photo_bar
• Twig Function: random_photo

This would make all these tools available to any UG that uses meetup.

[pascaldevink]

What would these functions do exactly?

[rdohms]

I'm thinking twig functions that obtain the data to be used to render whatever it is.
But that may or may not violate some nasty rules of encapsulation :P