Added property to QuadCurveRadialDirector for allowing enforcement of exact angles.

Right now when setting a whole angle of `M_PI` (`180˚`) the last menu
item will be shown at `angle * itemIndex / itemCount`. As a consequence
the last menu item is not shown at `180˚`, as one would expect. For
`~360˚` this makes sense, as it avoids overlapping of menu items, for
anything else it doesn't quite.
My modification keeps the current (odd) behaviour as default, but
allows menu item distribution by extact angles by setting
`radialDirector.spreadsToEntireWholeAngle = YES;`.
Updated Radial Director to default to use whole angle when less than 360

* Renamed the property
* Defined helper methods to assist with the determination