replace optparse with argparse

optparse is deprecated with Python 2.7.
This *should* replace it without changes to the behaviour.
But as I don't have experience using Frescobaldi with different
command line arguments I could not test it thoroughly.
small argparse usage improvements
argparse messages
new pot, po, nl translation
Merge pull request #499 from wbsoft/argparse

replace optparse with argparse
require python 2.7
reword action Close All Documents to say that it also leaves the session

close #500
small i18n update