Add two new methods : fetch_nearest_up & fest_nearest_down

Dear benwwbooth,

Thanks for your great work on this perl module to deal with interval queries, it is very efficient and easy to use. I have introduced two new methods (fetch_nearest_up and fetch_nearest_down) to quert the structure that would be very helpfull for my use of Set::IntervalTree.

Thanks for considering this pull request.

Best,
Jerome.