Faster load, better API doc, and some other small things

Some small improvements over the past two days to Da Button Factory:

• a bug in the undo/redo system was fixed, where the color fields were not properly reset to the right color
• the online button creator is now faster to load, thanks to a few tricks (fewer external javascript files to download, reduced CSS code, …)
• the API documentation has been updated & improved
• the Contact page now has a “Frequently Asked Question” section. Among other things, it says explicitely what the generated buttons license is: free to use and embed without restriction!
• and some other minor things :-)