added note about color schemes to HISTORY.txt
color scheme improvements

- shorten the example text
- fix a bug that would prevent info-based color schemes
  to be set properly when drracket first starts up
- make the preferences dialog examples use the user's
  preferred font, not the default font
- move the documentation and info buttons to the top
  of color scheme section of the prefs dialog
- added a checkbox to go back to the default version
  of the color scheme