Fix #21: Fix more W3C HTML validator issues

• credits.html
  • Use CSS to specify bcColor of table
• download.html
  • Use CSS <table border="1">. This looks like an approximation of what we had, but should be good enough
  • Remove extra column from the row 2009-11.
• search.html
  • Use CSS to align for <div> and <img> tags
  • Use CSS to set the border for an img and make sure the image is still centered
  • Replace <font> with <span>
• support.html
  • Replace <tt> with <code>

Run tidy over all the files.