Merge fix for #211 into v2 branch

Repeat of https://github.com/jakerella/jquery-mockjax/pull/213 to be merged into branch `v2`, plus other improvements discussed there.
This looks great, just one change, which is this return. If they submit an invalid `responseTime` I think we should return the default value. Of course, we can't just use `$.mockjaxSettings.responseTime` since they could have overridden that. Perhaps directly above the `$.mockjaxSettings` creation at the bottom of the file we can create a local private variable for the `DEFAULT_RESPONSE_TIME` and then use that in the initial `$.mockjaxSettings` and in this `return` statement?