add placeholder polyfill for IE9 support

source: https://github.com/mathiasbynens/jquery-placeholder
call $('input, textarea').placeholder() within `rendered` event

This adds a superclass to all views, in order to make sure this is always called. This trades off calling `placeholder()` unnecessarily in favor of not having to worry about whether the template for a view contains an input element or not.
Merge branch 'placeholder-polyfill', fixes #117