Adding SVG export support

[electronicwhisper]

Not done yet, but progress on SVG export (for laser cutting, printing, etc).

To do:

• Complete support for circles. Right now only pure circles are supported (no shear transforms or non-uniform scaling).
• Add support for text.
• Design UI for export, options around scaling (DPI) and bounds.

[electronicwhisper]

I have completed the implementation for circles and text. Images currently do not render in SVG.

@joshuahhh, if this looks good to you please merge it into master / the main website in order to make it easier for lab members to do laser cutting via Apparatus.

This is useable now by going in the console and doing Apparatus.editor.exportSvg() which will download an svg file. One can also do Apparatus.editor.exportSvgString() to just get the string (I'm planning on using this for a lasercutter microservice bookmarklet for the lab).

I opened #68 for a list of features remaining to bring this beyond "experimental" status.