Added allowPast settings & tests

Added a setting for ```allowPast``` that when set to "false" prevents timeago from showing times that are in the past, and instead displays a static message defined in ```settings.strings.inPast```. This was useful for a case where we had a count-down timer on a screen that used ```allowFuture=true``` but we did not want the displayed message to go to an "X minutes ago" message after the timer had reached 0. ```allowPast``` defaults to "true" which retains the existing behavior, and either ```allowPast``` and/or ```allowFuture``` must be set to "true". An error is thrown if they are both set to "false".