Make layout examples dynamic and improve route metadata with Express Map

This removes all the specific routes for each layout example and
replaces them with one, dynamic route: "/layouts/:layout/".

This also updates how the data for the nav menu is generated, and adds
a new `pathTo` Handlebars helper for writing URLs to routes, including
the new dynamic layouts route:

    <a href="{{pathTo 'home'}}">Home</a>
    <a href="{{pathTo 'layout' layout='pricing'}}">Pricing Layout</a>
Remove redundant view name and use the route's annotated name
Remove _even more_ redundancy in the routes definitions
Change order of arguments for `routePage()` sugar function
Remove "Built on Pure" section from the Extend page

These modules are not really built on Pure and it seems better to remove
the reference from them now util the Bower enhancements land and we use
that to extend Pure.

Closes #125
Enhance Grids docs with clearer constraints

Closes #118
Add `<meta name="description">` to all pages

Closes #109
Set a `max-width` on the main content so line lengths stay in check

This also adjust margins around the `.header`, `.content`, and `.legal`
classnames so they are lined up.

Fixes #108
committed by screwdriver build
Merge pull request #154 from ericf/dynamic-layouts

Make layout examples dynamic and improve route metadata with Express Map
Merge pull request #155 from ericf/remove-built-on

Remove "Built on Pure" section from the Extend page
Merge pull request #156 from ericf/grids-units

Enhance Grids docs with clearer constraints
Merge pull request #157 from ericf/meta-description

Add `<meta name="description">` to all pages
Merge pull request #158 from ericf/max-width

Set a `max-width` on the main content so line lengths stay in check
committed by screwdriver build
Regen npm-shrinkwrap file