Create a skeleton controller and show view for users
Add /:username route to user profile
Add rudimentary styling to user show view to make stub text visible
Check that the username is valid before showing the user profile
Create user#show view for a profile that displays the user's twets
Add helper to convert @username mentions to hyperlinks in the twet content

Now available for twets displayed in the user profile view. This functionality
needs to be propagated to other views that display twets.
Refactor user profile and twets index to use a shared view for each twet

Now all @mentions in both views will render links to that user's profile.
Refactor shared left nav to render stats for a specified user

Before this change, the left nav always rendered info and stats for the
currently-logged-in user. Now, the view receives a user object to be
rendered. This way, the view can be re-used by the user profile view.

Additionally, if the user to be displayed is not the currently-logged-in
user, the content area and submit button for new twets are not rendered.
Add the user's gravatar to the left nav panel