Add matchers for validating dates

[mscharhag]

Something like:

expect(date).toBeBefore(otherDate);
expect(date).toBeAfter(otherDate);
expect(date).toBeBetween(first, second);
...

[mscharhag]

Added the following methods in 01850df

expect(date).toBeBefore(other);
expect(date).toBeAfter(other);
expect(date).toBeCloseTo(other, 1000); // second parameter is delta in ms
expect(date).toBeBeetween(other);
expect(date).toBeNull();
expect(date).toBeNotNull();
expect(date).toBeEqual();