Root-level component no longer anchors directly to <svg>

Instead, if a user tries to anchor a component to <svg>, the component
creates a <g> under the svg and anchors to that instead. The <svg> receives
the CSS class "plottable".

Also removed sourcemap for tests because the sourcemap was interfering with
test debugging (debugger wasn't following them properly).
Proxy SVG dimensions to root-level <g> instead of storing it in variables

Makes more sense to put it there.
Merge pull request #153 from palantir/root-node-anchoring

Root-level component no longer anchors directly to <svg>