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 divisble by 400.
For example, 2001 is a typical common year and 1996 is a typical leap year, whereas 1900 is an atypical common year and 2000 is an atypical leap year.