Sorted dict 1

Pull request for SortedDict and BalancedTree23 taking into account Kevin Squire's feedback from the past 12 days.
Hi Stephen,
A code coverage tool was hooked up to this repo's Travis runs 12 days ago (if you click the coverage badge, the image that says 95%, on the github readme, it'll take you through to code coverage breakdowns by pull/file/line).

Rebasing this branch off master would bring the travis config for the branch up-to-date and give you those nice visual coverage statistics (watch out for lines that aren't green or red -- they may or may not have coverage -- (hopefully temporary) shortcoming of the tool having to do with compilation and inlining).

Alternately you can run `Pkg.test("DataStructures", coverage=true)` or `julia --code-coverage test/runtests.jl` locally, which will fill your src/ directories with `.jl.cov` files that show which lines are covered (the warning about non-highlighted lines above applies to lines beginning with '-' instead of a number here).