transactions.js: add method to get event from transaction

Pass a transaction and it will return the associated event.
collections/users.js: add methods to get associations

* Method to get transactions for a user
* Method to get total points for a user

The user id needs to be passed into both methods.
checkPoints.js: update helpers to use new users collection methods

Get transactions and total points from the users collection.
Merge pull request #19 from drenfr01/feature/move-model-functions-to-collections

Feature/move model functions to collections