Sorted Containers

Added sorted containers to DataStructures, testing, and documentation.
Replaced README.md with README.rst
Because we typically want numeric keys to be convertible to one another, the typical way to write this is

```julia
function cmp2{K}(treenode::TreeNode{K}, _k, isleaf::Bool)
    k = convert(K, _k)
```

If `_k` happens to be the correct type, this is a no-op.