Russian-specific plurals support added.

In Russian language there are several forms of plurals:
 - 1 cat - 1 кошка (form 'one' used)
 - 2,3 or 4 cats - 2, 3 или 4 кошки (form 'two' used)
 - 5-20 cats - 5-20 кошек (form 'five' used)
 - 21 cats - 21 кошка (again form of 'one' used)

... and so on.

Also, added 'zero' key for more human readable outputs for 0 case (like 'there are no monkeys in the tree').