Several improvements, some of where were contributed by Bernhard

Herzog <herzog@online.de>.  Specifically,

--verbose/-v flag added

pot_header added to make msgmerge and Emacs po-mode work better

normalize(), escape(), safe_eval(): Improved normalization of strings
for more .po file compatibility (e.g. C style).  Handles emmbedded
newlines better.

Also added an identity function called _() and use it in the file
where messages are printed.  This allows us to selftest pygettext.py
with itself as input.