Merge pull request #7 from fred-wang/localization

Translate more French strings and escape the non-ASCII chars.
Change URL for MathJax font download to point ot GitHub.
Add Help dialog box (rather than link to a site).  Fix Markdown to substitue values for URL's in links.
Add HelpDialog to the list of domains for the three languages, and make sure that the domain file is loaded (even if the language is changed after the help dialog has been viewed in another language).
Translate HelpDialog in French ; fix a typo in the English version ; add "MathJax Help" to the HelpDialog domain.