Major code cleanup, full PEP8 compliance, replace %-formatting with .format()

All changes:
- import the print function
- replace the %-formatting with .format()
- full PEP8 compliance
- fix problem when latest string in file has a plural form (this last
  translation was ignored)
- use codecs module to unescape strings (faster with python 3)
- display full exception in case of problem when reading file
- add short option synonym '-P' for '--pwl'
- add short option synonym '-e' for '--extract'
- rename some long names for command line options:
    --compile         -->  --no-compile
    --lines           -->  --no-lines
    --punct           -->  --no-punct
    --whitespace      -->  --no-whitespace
    --onlymisspelled  -->  --only-misspelled
- major code cleanup: add comments, move checking/error functions from
  class PoMessage to class PoFile