Add lint friendly assertions

Starting in chai 1.10.0 there are callable assertion terminators available; see chaijs/chai#297. This change checks for this functionality and uses it if available.

These both will now be valid ways to assert for `called`, `calledOnce`, `calledTwice`, and `calledThrice`.

    spy.should.have.been.calledOnce;
    spy.should.have.been.calledOnce();