Added extracting plural forms using standard "n"-prefixed gettext functions

[vrouet]

This adds basic plural forms extraction, it suits my need well, it'd be great to have merged. I'm sure there are stuff I didn't think about and that would need fixing before, so please comment on this.

[vrouet]

I've noticed that one test if failing, but I checked on the master version and it does fail as well, so it's not something I broke ;)

[BYK]

First of all this is great! Thank you since we have been wanting this for quite some time now. I have a few requests though:

1. Please add some tests
2. Squash your commits
3. May be put an example usage/expected format somewhere since even I am not very familiar with ngettext usage. Adding tests may cover this.

[BYK]

Fixes #51

[vrouet]

Thanks for your reviews, I will definitely add some tests and doc for that, I made the pull request first basically to see if my approach was fitting your standards, and did rthings quickly because I needed to tel my dev team what to use for i18n on our project. I will update soon

[vrouet]

Ok, I have taken your comments into account, added some notes, integrated the plural functions to all of the relevant test cases, and squashed my commits

[BYK]

And rebase to get tests fixed :)

[vrouet]

Alright, rebase is done. Nice to see the whole test suite passing ;)

[BYK]

Alright, rebase is done. Nice to see the whole test suite passing ;)

Definitely. You triggered me to do it :)

Final thing before merge: fix those %s lines in the README please?

[vrouet]

OK now

[BYK]