Added new test methods lessThanOrEqual() and greaterThanOrEqual()

[nitram509]

Hi,

while writing tests for a graph layout algorithm, I've needed to more test methods, like

expect(3).to.be.belowOrEqual(9);
expect(5).to.be.aboveOrEqual(2);

So I've added them to expect.js library.
Tests are included.

Regards
Martin

[EvanHahn]

My fork Expecting.js has this change, if you're interested.