Generate Python API reference with Sphinx

- Adds docs/sphinx-api; contains html/pdf/clean ant targets and html/latexpdf/clean make targets
- Adds top-level release-sphinx-api target

This is intended to replace the release-epydoc target and all use of epydoc once we are satisfied that it is good enough to replace it.  This PR adds the initial support to generate the API reference, but does not mark up the python sources with sphinx markup; this will need doing separately.

Testing:
- run `build.py clean build-default release-sphinx-api`; check the output in `dist/docs/api/python` or `docs/sphinx-api/_build`
- in `docs/sphinx-api`, run `ant clean html pdf` and `make clean html latexpdf`
- compare the epydoc docs to the sphinx docs; is anything missing (not including the actual docstring content)?

Things to consider:
- are the target names OK
- is the dist dir correct?
- does the root index need any more content?
- do we need a `Makefile` or is ant sufficient?
- do we need a PDF or is HTML alone sufficient?
- can we reuse any of the sphinx config from docs/sphinx?
- anything I've missed which needs doing?