Sorted Containers

Added sorted containers to DataStructures, testing, and documentation.
Replaced README.md with README.rst
It is actually discussed [here](http://julia.readthedocs.org/en/latest/manual/control-flow/#repeated-evaluation-loops), although it wasn't in the Standard Library documentation.  I added a brief mention of it just now, which should be propagated to the `v0.3` and `latest` docs within a day-ish.