outputarea.js: Wrap inline SVGs inside an iframe

When multiple inline SVGs are included in a single document,
they share the same parse tree. Therefore, style collisions and
use id collisions can occur and upset the rendering.

This patch wraps each SVG inside an individual iframe, ensuring
that SVG's declarations do not collide.

(The SVG representation is kept as XML and not converted to a binary
format, so I do not think this approach precludes the use of d3.js)

Tested on:
* Chrome Version 29.0.1547.57 Debian 7.1 (217859)
* Mozilla/5.0 (X11; Linux x86_64; rv:17.0) Gecko/20130806 Firefox/17.0 Iceweasel/17.0.8

Closes #1866
SVG scoping must be explicitly enabled by the user

SVG scoping is disabled by default, to enable it the user
must call the core.display.SVG constructor with the scoped=True
keyword parameter.