Sorted Containers

Added sorted containers to DataStructures, testing, and documentation.
Replaced README.md with README.rst
Okay, there were fewer than I thought.  That said, I'm wondering if all of these need to be exported/documented?  Particularly, it's unclear how useful `is_ind_past_end`, `before_start`, `is_ind_before_start`, `advance_ind`, and `regress_ind` are (in Julia).  Can you offer some justification/use case (beyond their existence in C++)?