prompt.history() fix

The way I read it, prompt.history() is broken. It doesn't look like it's being used internally, nor are there tests for it, which is why this has been missed.

The problem line is here: https://github.com/flatiron/prompt/blob/master/lib/prompt.js#L147 

.. which reads: "If the property name exists in the list of properties then return null.", when it should be "If the property name **does not exist** in the list of properties then return null."

I've added a fix for this, as well as added a test which tests prompt.history() and have updated the README.

Please note the extra changes are due to trailing whitespace, which I've removed.