Context nesting

Created by: bhuga

This is an updated version of https://github.com/rspec/rspec-core/pull/240, which allows contexts to stay nested across files with the HTML and documentation formatters. I've been using it for a week and have noticed nothing awry, so this would hopefully be a less spastic pull request, updated less frequently than every 20 minutes.

I will squash this into one commit if you prefer. The issue is #238 (closed).