Reworked documentation building to work with extra modules.

The main problem with extra modules is that they're located outside
of the OpenCV root, while Sphinx requires that all documents are
within the documentation root. To circumvent this, we create a
"fake root" and copy all documentation (as well as all files that
it depends on) there. It's a bit messy, but what can you do.

As a bonus, this eliminates the need to generate modules/refman.rst
inside the source tree.
Merge pull request #1793 from SpecLad:external-docs-1