Adds some simple styling based on pure-css
* CSS classes added to all forms and buttons
* pulls signup and signin links to header menu
* pure.css stylesheet added to vendor/assets
* require pure.css in application.css
Adds content-tag based helper to set current menu item class to pure-menu-selected
Also ammends application.html to include this tag helper
Merge pull request #24 from tiger-swallowtails-2013/spike-pure

Spike pure