python 3 porting: syntax adaptations

These changes fixes the usual syntax/import differences. I still get this error under python 3 though, not sure why:

    Traceback (most recent call last):
      File "packular.py", line 284, in <module>
        main()
      File "packular.py", line 276, in main
        targets = configure()
      File "packular.py", line 152, in configure
        return read_config(config, defaults)
      File "packular.py", line 116, in read_config
        for sect in cfg.sections() if sect.startswith('target:'))
      File "packular.py", line 116, in <genexpr>
        for sect in cfg.sections() if sect.startswith('target:'))
      File "/usr/local/lib/python3.4/configparser.py", line 802, in getboolean
        raw=raw, vars=vars)
      File "/usr/local/lib/python3.4/configparser.py", line 776, in _get
        return conv(self.get(section, option, **kwargs))
    TypeError: get() got an unexpected keyword argument 'vars'

Anyways, I think you could start merging this into master.