LPS-36328 - Update Welcome theme to use bootstrap CSS

Hey @natecavanaugh,

Attached is an update for http://issues.liferay.com/browse/LPS-36328.

NOTE: I had to refactor some of the CSS to work with Bootstrap's grid layout.  Also because of the HTML5 elements, the <header>, <footer>, and <nav> elements do not render correctly in IE < 9.

Please let me know if you have any questions.  Thanks!