Write a function that returns true or false depending on whether its input integer is a leap year or not.
A leap year is defined as one that is divisible by 4, but is not otherwise divisible by 100 unless it is also divisible by 400.
For example:
2001 is a typical common year
1996 is a typical leap year
1900 is an atypical common year
2000 is an atypical leap year.