Encourage override of toBBox, compareMinX, compareMinY for CSP issues

Where the Content Security Policy (or other) disallows the use of eval and related functions (e.g. browser extensions), the toBBox, compareMinX, and compareMinY methods can be overridden.
Merge pull request #14 from tschaub/less-evil-II

Encourage override of toBBox, compareMinX, compareMinY for CSP issues