Add expect_to_throw() feature and should()… an alias of it()

Adds a new `expect_to_throw()` function that is used to tell pecs that you're expecting the spec to throw an exception. Saves needing to wrap this kind of code in a callback function.

Ideally I would provide a test case which I expect to fail, but not sure of how to do that with pecs.

I also made should() alias it(), so instead of having to do `it("should add a location with a city", function () {…` I can write `should("add a location with a city", function () {…`.