Hebrew dual form fixed

In Hebrew, there is a special dual form for times like 2 hours, 2 days, 2 months and 2 years.
Not using this, is the biggest *tell* for a non-native speaker (or a machine).
The dual form for minutes and seconds isn't used in Modern Hebrew.

[Wikipedia: Dual (grammatical number)](http://en.wikipedia.org/wiki/Dual_grammatical_number#Modern_Hebrew):
> Modern Hebrew: The dual form is usually used in expressions of time and number. These nouns have plurals as well, which are used for numbers higher than two

I've implemented this dual form as discussed in [issue #488](https://github.com/moment/moment/issues/488), but never actually implemented.

I've also made sure of complete test coverage for this.