Add RSpec tests for models and controllers

Adding model and controller specs.
I think that the description in the `context` should describe an already existing state,
and not the steps that you will execute. In other words, this is ok:

> context "when the user has a standup for today"

Your text is more appropriate for a `describe` block. For example:

> describe "creating another standup the same day"

or even better

> describe "creating more than one standup for the same day"