Aggregate whats_new and api_changes rst files for docs

[tacaswell]

We are currently collecting whats_new and api_changes in separate rst files which we did to avoid the rebasing/manual merging pain we had in the run up to the 1.4 release, but to build the docs for 1.5 we need to merge all of these back into a single rst. The options I see are:

• do it manually
• adapt IPython whats_new scraping script, do it permanently, automated commits and all
• do it on the fly at documentation build time
• sort out how to get sphinx to auto-include a bunch of files

This seems like a good project for the sprints.