weercd.py: major code cleanup, full PEP8 compliance, replace %-formatting with .format()

All changes:
- import the new division operator and the print function
- replace the %-formatting with .format()
- full PEP8 compliance
- major code cleanup: new function send_cmd() to send an IRC command,
  split flood() function into many functions.

The messages sent from a file (or stdin) are formatted with
".format(self=self)", so that attributes from class Client are replaced
with their values. You can use for example these attributes:

  {self.nick}     current client nick
  {self.name}     the server name ('weercd')
  {self.version}  the server version (version of weercd.py)