Flexbox support

Attempting to get away from floats and migrate to flexbox where it's supported.

* Does _not_ support the legacy flexbox syntax (see http://css-tricks.com/old-flexbox-and-new-flexbox/ for info)
* Requires either Modernizr.js or another method of testing modern flexbox support that adds a class of `flexbox` to the <html> tag
* Only adds the flexbox rules if `$flexbox_support` is set to true either in `smart-grid-variables.scss` or in your custom scss file.
* A `row` class is required to trigger the flexbox method. This can be on the same element as the `container` class. Additionally, if you want to add the syntax to your own selectors instead of adding non-semantic markup, `@extend %flexbox_row` on the selector.

This has been tested in the latest Chrome, Safari, Firefox (which does _not_ support the modern flexbox syntax) and IE 10 (both Desktop and Metro UI versions). If you find any bugs please file an issue.