Add SVGImage loading & parsing functions
Add SVGImage loading & parsing functions

This commit adds two public and static functions for loading an SVG
image:
- fromString to load from an XML string
- fromFile to load from an XML file

The loaded data is then parsed into nodes and an instance of SVGImage
containing these nodes is returned.

TODO:
- Add support for styles (in style attribute but also as standalone
attributes)
- Handle child document fragments

JangoBrick/php-svg