Improve unicode support

• Fix handling of b-strings under Python 3 #18
• Fix handling of source file encoding #18
• Fix handling of generic unicode string #18
• Fix handling of unicode identifiers #18
• Add a hotspot attribute to base widget used for dragging #17
• Generate lex/parse tabs in setup.py so that they are always up to date and
  correctly removed during unistall (future is now a setup dependency) #23