use And to add more expectations to a Then

Hi again.  To be able to chain `Then`s without needing parentheses, I got `And` working with `Then`

    Then -> timesGivenWasInvoked == 1
    And -> timesWhenWasInvoked == 1
    And -> timesGivenWasInvoked == 1
    And -> timesWhenWasInvoked == 1

I took the liberty of removing the doc of the paren-chaining version from the README, but it's retained in the code and specs for backwards compatibility.