CLDR & Gwt properties file handling

It's not clean commits but it's just for the idea.

Currently the order of plural forms is the one from gettext and there is no way to know which plural form is the first,second, entry in the multistring.

In GWT the plural froms are derived from CLDR. This pull request introduce some cldr mapping.
for exemple:
'fr': (u'French', 2, '(n > 1)', ['one', 'other']),
'ar': (u'Arabic', 6, 'n==0 ? 0 : n==1 ? 1 : n==2 ? 2 : n%100>=3 && n%100<=10 ? 3 : n%100>=11 ? 4 : 5', ['zero', 'one', 'two', 'few', 'many', 'other']),
Lot of mappings are missing
Which this mapping we can identify each plural forms and convert this forms in GWT properties format

This pull request is linked to another pootle pull request (less linked to GWT stuff)