Sorted dict 1

Pull request for SortedDict and BalancedTree23 taking into account Kevin Squire's feedback from the past 12 days.
That's weird for two reasons: (1) the missing .cov files for "user" code, and (2) the fact that there _are_ files for base. `--code-coverage`, by itself, isn't supposed to generate files for base (you'd need `--code-coverage=all`). But, in this case I think you're turning `code-coverage` on twice, in two separate julia sessions (once by the command line option, and once in the process that gets launched by `Pkg.test("foo", coverage=true)`). Try picking just one of these and see what happens.