Sorted dict 1

Pull request for SortedDict and BalancedTree23 taking into account Kevin Squire's feedback from the past 12 days.
Only the 2nd works for me. The issue is that `Pkg.test("foo")` starts a new julia process, and that 2nd process won't have `code-coverage` turned on. I'm really puzzled about why you got _any_ `*.cov` files in that case. (If you're going to use the command-line variant, you have to run the tests with `include("runtests.jl")`.)

But since the 2nd works for me as expected, maybe it is a platform-specific bug.