rxgettext - added --gnu-compatibility option

[lslezak]

to produce a GNU gettext compatible PO file (no double escaped new line charaters, removed Plural-Forms header tag)

Small implementation change: subclass RubyPOEntry from POEntry to reuse the format_message method and to avoid aliasing initialization method.

[lslezak]

BTW there is a small overlap with #16.

[lslezak]

The reason for this change is that our translators already use GNU tools and we need to produce backward compatible POT files after switching to Ruby. I added a new command line option so it does not break the current behavior for rxgettext and rmsgfmt tools.

[375gnu]

lslezak, is this a good idea to remove Plural-Forms? I think today 99% of po files are made using gnu gettext and they all have Plural-Forms.

[lslezak]

I have added it back, we merge POT files created by both rxgettext and GNU gettext and during merge msgcat complained about invalid nplurals value.

I have fixed our merging script and reverted the removal.

[kou]

Thanks!
I've fixed it in master.

Sorry for my late fix...