New "side-menus" component.

Based on the `modals` component.

See example at http://michaelrienstra.com/temporary/ratchet/side-menus.html

The current implementation relies on putting a wrapper (`<div class="side-menu-siblings-wrapper">`) around the `side-menu`s siblings (`<header class="bar">` & `<div class="content">`). I have working code to handle the wrapping of siblings automatically, as well as working code that works without the wrapper (but I realized I was going to have to add more "moving bits" to get the subtle drop shadow to animate without the wrapper).

Minimally tested on Android (Nexus 4) & iOS (iPhone 5).

Happy to do more testing / etc. if this component is of interest.

Attribution: I borrowed a few lines of CSS from http://mmenu.frebsite.nl/ (which is dual licensed under the MIT and GPL licenses).