Added justDown access to _justDown internal boolean.  Records whether the key has been pressed since the last time it was checked.  Resets to false only when it is read.  Used for anything that needs to know when a key is first pressed with no autorepeat, without using closures or callbacks.
Merge pull request #1321 from pjbaron/dev

justDown property indicates if a key has been newly pressed since last time you checked