Sorted Containers

Added sorted containers to DataStructures, testing, and documentation.
Replaced README.md with README.rst
Some of the functions below overlap with functionality in base, and so should probably be renamed to match the functions in base.  Unfortunately, those names might change at some point (see https://github.com/JuliaLang/julia/pull/7327), but we can adjust if/when that happens.  I'll comment on individual functions below.