Fix semantics of html markup in MVC sample pages

This PR proposes couple of small changes to MVC sample pages markup:
- adds textual context to toggle button to improve accessibility:
https://github.com/twbs/bootstrap/blob/master/docs/_includes/nav/main.html#L5
- remove empty CSS class from markup
- use WAI-ARIA role on anchor:
https://github.com/twbs/bootstrap/blob/master/docs/examples/jumbotron/index.html#L62
- fix negative margin introduced by incorrect markup nesting:
http://www.helloerik.com/the-subtle-magic-behind-why-the-bootstrap-3-grid-works

![row](https://cloud.githubusercontent.com/assets/14539/5559601/f836c062-8d53-11e4-911f-64114300a7d4.gif)

Let me know if you want to update or rebase this PR,

Thanks!