Cache constituent converters for GenericNodeConverter

Since GenericNodeConverter is used so often for the same document, it
makes sense to cache its constituents to reduce memory waste. This code
should be fine for the first pass but will eventually need to be
modified to remove constituent sets from the cache when it's finished
with them.
Added functionality to SVG line converter

Added start and end points to SVG elements produced by the line
converter. Also added basic support for stroke and fill for all shape
nodes.
Implemented FXConverter methods in FXToSVGConverter

Finished the last piece of a rough implementation of the SVG converter
by adding code to actually instantiate the document and create the svg
tag.