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
Docs update.
Key.justPressed has bee renamed to Key.downDuration which is a much clearer name for what the method actually does. See Key.justDown for a nice clean alternative.
Key.justReleased has bee renamed to Key.upDuration which is a much clearer name for what the method actually does. See Key.justUp for a nice clean alternative.
Key.justDown allows you to test if a Key has just been pressed down or not. You can only call justDown once per key press. It will only return `true` once, until the Key is released and pressed down again. This allows you to use it in situations where you want to check if this key is down without using a Signal, such as in a core game loop (thanks @pjbaron #1321)
Key.justUp allows you to test if a Key has just been released or not. You can only call justUp once per key press. It will only return `true` once, until the Key is pressed down and released again. This allows you to use it in situations where you want to check if this key is up without using a Signal, such as in a core game loop (thanks @pjbaron #1321)