Implement <meta name=viewport> handling
Translate <meta name=viewport> as according to [CSS Device Adaption § 9](http://dev.w3.org/csswg/css-device-adapt/#viewport-meta)
Note: as the PR currently stands, handling `<meta name=viewport>` elements always occurs. This is probably not desired for some contexts (e.g. desktop), but I'm unsure of how to conditionally handle elements based on that.
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This allows both boolean and string-type preferences. It
also implements a system where prefs that are read from a
configuration file can be reset back to their initial value,
which is useful in a number of cases e.g. when running tests
to ensure that each test starts with the same values for
the prefs.
This property determines the background positioning area, that is the position of
the origin of an image specified using the 'background-image' CSS property.
'background-origin' is ignored when background-attachment is fixed.
Spec: http://dev.w3.org/csswg/css-backgrounds-3/#background-originFixes#6045.
This is for parsing of the rule within a stylesheet only. <meta name=viewport> could create a viewport rule and add it to the list of stylesheets for the page (like quirks mode).
