Bug 891835 : extract comments in pot files into lang files

* gettext.py/pot_to_langfiles() updated to extract comments from pot file to templates/*.lang files
* gettext.py/_append_to_lang_file() now outputs comments if they exist
* dotlang.py/parse() has a new optional parameter: extract_comments=False
* l10n_merge uses extract_comments=True
* updated test_merge_lang_file()
* added test_parse_with_comments()
* all tests pass