Make 2d comparison functions consistently use _x style properties

Modifies isAt, contains, and within to accept objects with _x style properties, such as MBRs.  (Fixes #504).  Would break code that passed manually constructed objects with old-style properties.

Adds a couple tests to better catch issues with MBRs and within/contains.
Merge pull request #505 from starwed/fix-2D-MBR

Make 2d comparison functions consistently use _x style properties