Add accessors to Rect attributes

Hi Daniel,

I have a need to compare the `Rect`s stored in a Rtree with another `Rect` in my application, so that I can avoid needless update of the Rtree when the contents are actually equal.
I'm aware that we want to make the `Rect`s immutable in order to maintain internal consistency within a Rtree, but it seems that it's also possible to for users to get the contents of a `Rect`, too. This pull request implements this by adding accessors/getters to the Rect's attributes while maintaining that the `Rect` remains immutable.