Benefits layout; /enterprise, /ecommerce, /small-business, /publishers pages

@kylerush this creates a new benefits layout to be used for /enterprises, /eccommerce, /benefits, /publishers, and /small-business. Also creates /enterprises page which uses this new layout.

The sidebar form is a simple include that lives in new partials folder (website-guts/templates/partials)

Questions:
1. In the top nav menu ("Optimzely for:"), see how dropdown menu items change on different pages (e.g. https://www.optimizely.com/ecommerce and https://www.optimizely.com/enterprises). How do we want to handle this in the layout?
2. How should we handle layout-specific CSS and JS? Right now I'm including them on a per-page basis, but it would be nice if we could just put them in the YFM of the layout so child pages can inherit them.

Related issues: #11, #10, #14, #15, #16 
@acreighton i'm thinking we opt for a javascript solution so that we don't have to enter these on each page. if we do it this way, when the url to one of these pages changes we have to change it in multiple places. i'd rather this yfm list be in the layout than the actual page. make sense?