Install Bootstrap; spread some CSS stylin' love for aesthetic loviness.

* put sign up and sign in fields into a well
* add uniform flash messages for app
* make Devise's flash message conform to app
  via a helper
* make header a partial
* make flash messages a partial
* fix width of header

Things that have nothing to do with styling
that I didn't bother to make a new branch for:
* add .DS_Store to .gitignore
* remove .DS_Store from repo