Ignore styles on foreign hosts.

Fixes #17.

- Add test page stylesheet link to foreign host to test failure under Firefox.
- Refactor external URL check into isExternal function.
- Warn instead of throwing an error when trying to embed images from foreign hosts.
- Warn when trying to get styles from foreign hosts.
Merge branch 'gh-pages' of github.com:exupero/saveSvgAsPng into gh-pages