Create thread model

Creating a model to contain an entire discussion. As it turns out, we can't use the class name Thread (it's reserved and I don't feel like namespacing the app yet), so I'm calling it a topic. 
Fabricator is a gem that makes it easy to create objects (FactoryGirl is another one, but I've had problems with that in the past and wanted to try something new). It lets you define default values for fields, create associations between objects, etc. I'm not using it yet, but we're going to be doing a decent bit of stuff where objects are related to each other (e.g. all the posts that have parents/children) so I think this will be useful later.