Add 'thrice' expectation.

Added a `thrice` expectation to complement `once` and `twice`. It's a perfectly good English word -- one of my favourites, in fact -- and I was mildly surprised when it didn't work.

Note that I didn't change the output for error messages ("expected foo at least 3 times", etc.), in case anyone's behaviour depends on that. 
Can you add back an `exactly(4).times` so this spec covers the same as before?