Deprecate rmagic

The R magic is in released versions of rpy2 now, and we're starting to see test failures because of changes in rpy2. This loads rmagic from rpy2 if possible, and removes our copy of the tests and docs.

Ping @davclark
Because of the cross reference: its target is being removed, which would result in a Sphinx warning when the docs are built. I'm keen to keep our docs builds clean so we can spot real problems when they arise.