fix validation errors

[pkra]

http://validator.w3.org/check?uri=http%3A%2F%2Fmathjax.github.io%2FMathJax-website%2F&charset=%28detect+automatically%29&doctype=Inline&group=0

• add img alt tags
• remove width from a's in supporter links
• remove name from sponsor headings
• pledgie remove border attr
• flash embed (via #4 )

[pkra]

Fixes (although the validator incorrectly complains about the CDN snippet popover code)