[python] unicode strings

[Patrick Totzke]

Unicode handling fixes for the bindings:
 - use __unicode__ for string representations, __str__ falls back to this
 - less complicated __str__ for Thread and Message
 - use errors='ignore' parameter for str.decode(). This should fix the
   "UnicodeDecodeError: 'utf8' codec can't decode byte 0xe8 in position"
   exceptions people were seing.

Best,
/p