Pluralizer Enhancements

The attached patch adds several enhancements to the pluralizing backend.

Firstly it passes the entry to the locale specific pluralizers as well as the count, so that they can check whether keys exist and do fallback much like the current handling of the :zero key.

Secondly it enhances the :en pluralizer to fall back to :other if :one does not exist - sometimes there is no need to special case the number 1 in English (consider "he had {{count}} sheep" for example).

Finally it adds a :sl pluralizer for Slovenian, which apparently sometimes requires special cases for numbers which (mod 100) are 1, 2 or 3/4. These also fall back to :other as they are not also needed. This pluralization logic is based on a user contribution to the translation of the http://www.openstreetmap.org/ site.