Handling Mozilla plural forms in properties files

Adds support for converting between Mozilla style and Gettext plural
forms.

Mozilla plurals is just a hack slapped on top of existing properties
files with no robust way to detect them, so we use a heuristic that
seems to catch the largest number of strings with no false positives,
but there are some false negatives.

No attempt is made to validate the forms.

Part of bug 329
Add --language option to moz2po

To be used for setting plural header.
Set nplurals when converting Mozilla to PO
Handle languages where Mozilla has different nplurals

This is likely to be a temporary measure, most of these languages are
just using the default value.
Add a very basic test for Mozilla plurals
Fix test failures
oo → Mozilla
explode → concatenate
Extend Mozilla plurals text

Check for:
    * more exception cases
    * correct handling of targetlanguage
    * the extracted strings
Test back conversion to Mozilla plural strings
[po2prop] Handle Mozilla plurals for unknown (to us) languages
Minor
Set X-Project-Style to "mozilla"

We will use that to avoid messing with nplurals in the header as it
would differ between Mozilla and Gettext.
Use Mozilla plurals from translate.lang