Integration with completion-at-point

Here, like discussed at https://groups.google.com/forum/#!topic/rope-dev/3cw9CpkbE4s.

The first commit is the main thing, so feel free to merge just it if you're feeling conservative.

The rest of them remove the `rope-code-assist` binding, which IMO works worse even than `completion-at-point`, and documents that change.