Use the "factory" method to get keys.
Rescue from NoMethodErrors when objects "lie" about responding to <=>.

Fixes #19.
Bump TreeNode threshold.

Using TreeNodes means going into Ruby-land, which is actually slower than extra long Node linked lists.
Merge branch 'chmv8-array-key-fix' of github.com:thedarkone/thread_safe into thedarkone-chmv8-array-key-fix

Conflicts:
	test/test_cache.rb