Add micropost model
Add validation - micropost must have a user_id
Set up associations between users and microposts
Make sure the posts are ordered from newest to oldest
Destroy posts when the associated user is destroyed
Add validations for micropost

* The content can't be blank/empty
* The content can't be more than 140 characters
Display user's microposts on their profile page

with styling and tests
Generate microposts with rake task

This task now generates users and 50 posts for the first few users.
Allow signed in users to delete microposts

Also refactor signed_in_user filter to make it available to both
controllers.
Generate integration test file for microposts
Allow a signed in user to create a new post

The new post form is on the home page.

Also refactor the error messages to allow a micropost object in
addition to a user object.
Display the signed in user's feed on the home page
When the post is invalid, use an empty array
Allow current user to delete their own posts