Ignore property changes on non-configurables

In Safari, properties of hosted objects (particularly DOM elements) are
not configurable so observing a property on a DOM element, even if to
traverse to an observable object, would fail with an error. This change
allows observers to proceed on the assumption that the non-configurable
property will not be changed.
Update find methods to account for wrong assumption about splay

splay does not guarantee that root.value <= value if value doesn't exist in the tree. Update comments and find methods to handle the case where root.value > value after splaying.
Add tests for SortedSet find* methods
Merge pull request #73

Fix SortedSet find methods.