Create a function that determines whether or not a given year is a leap year. Leap years are determined by the following rules:
>There is a leap year every year whose number is perfectly divisible by four - except for years which are both divisible by 100 and not divisible by 400. The second part of the rule effects century years. For example; the century years 1600 and 2000 are leap years, but the century years 1700, 1800, and 1900 are not.
```javascript
leapYears(2000) // is a leap year: returns true
leapYears(1985) // is not a leap year: returns false
