Create model to represent a specific edition of RailsBridge

[sophiedeziel]

The idea is to link registrants to a specific edition of RailsBridge. Right now, they all are part of the 2014th edition. We want a way to know who attended which edition.

• Add registrations to the currently open one
• Display informations about the currently open one
• When none is open, waitlist the regitrations

[sophiedeziel]

#112 is the first item of the checklist