Added detection of held keys.

Code added to the `input` method of `PygcurseSurface` to detect held keys. New parameters added - `delay` being the delay before held keys are detected and `repeat` being the frequency at which a held key sends a key press event to the `inputObj` of the `PygcurseSurface` object. One minor backwards incompatible change - default fps set to 60 - a fixed fps was needed to determine how often a held key should send a key press event (the relevant line is `if counter >= fps//repeat:`).