MVP Fixtures

I added our collections and fixture data (aka seed data). It's all working on my local Mongo, and we'll have to see how it renders. A few notes:
- I kept dates in "Epoch time" in the date fields but we'll have to ask if there's a better form for the date.
- I'm not sure whether Mongo/Meteor automatically applies timestamps like in rails. May have to check
- On each collection, I added an id (e.g. messageId in Messages collection). This id shouldn't be used in making calls to the collections—we should use the collection's _id instead. I made this other id just for the purposes of making associations.