Add polyfill service for missing DOM methods

This allows gemini client script to run in older browsers, that have no
support for `getComputedStyle`, `matchMedia` and `querySelector` methods
(for example, IE8).

Polyfill.io service is used as a library for this purpose.

This commit still does not allow gemini in IE8 because of 2 issues:

1. `getBoundingClientRect` in IE8 does not contain `width` and `height`
properties.
2. Full page screenshot contains border.
Create Rect class for client scripts

Main purpose of this class is to allow working with IE8 bounding client
rect (does not contain width and height properties and not
polyfillable).

Additionally, it encapsulates all rect-related functions, so region and
coverage scripts are now much more consistent.