html_validation gem

Used [`html_validation`](https://github.com/ericbeland/html_validation) gem.

Output of an error:

```
  1) Valid HTML5 Member is not signed up I do not exist as a member -> I sign in with valid username and password -> I see an invalid login message -> I should be signed out -> the page should be valid HTML5
     Screenshot: /Users/josh/Documents/Work/MuheimWebdesign/transition/tmp/capybara/screenshot_2014-01-02-22-13-57.079.png
     Failure/Error: And the page should be valid HTML5
       example.com/members/sign_in Invalid html (fix or run 'html_validation review' to add exceptions)
       example.com/members/sign_in exceptions:
        line 1 column 238 - Warning: <header> proprietary attribute "asdf"
       line 1 column 238 - Warning: <header> proprietary attribute "role"
       
       
        <!DOCTYPE html><html><head><title>Transition</title><link data-turbolinks-track="true" href="/assets/application.css" media="all" rel="stylesheet" /><script data-turbolinks-track="true" src="/assets/application.js"></script></head><body><header asdf="123" id="header" role="asdf"><aside id="mini_hub"><p>You are not signed in.</p><ul><li><a href="/members/sign_in">Login / register</a></li></ul></aside></header><p id="alert">Invalid login or password.</p><h2>Sign in</h2><form accept-charset="UTF-8" action="/members/sign_in" class="new_member" id="new_member" method="post"><div style="margin:0;padding:0;display:inline"><input name="utf8" type="hidden" value="&#x2713;" /></div><div><label for="member_login">Login</label><br /><input autofocus="autofocus" id="member_login" name="member[login]" placeholder="Username or email" type="text" /></div><div><label for="member_password">Password</label><br /><input id="member_password" name="member[password]" type="password" /></div><div><input name="member[remember_me]" type="hidden" value="0" /><input id="member_remember_me" name="member[remember_me]" type="checkbox" value="1" /><label for="member_remember_me">Remember me</label></div><div><input name="commit" type="submit" value="Sign in" /></div></form><a href="/members/sign_up">Sign up</a><br /><a href="/members/password/new">Forgot your password?</a><br /><a href="/members/confirmation/new">Didn&#39;t receive confirmation instructions?</a><br /><a href="/members/unlock/new">Didn&#39;t receive unlock instructions?</a><br /></body></html>
     # ./spec/support/steps/global_steps.rb:6:in `block in <top (required)>'
     # ./spec/acceptance/members/valid_html.feature:9:in `the page should be valid HTML5'