Use ant to build sphinx docs

With this PR, make is no longer required to build the sphinx docs.

- in docs/sphinx, build.xml provides `html`, `pdf` and `clean` targets, plus `latexpdf` for make compatibility
- toplevel provides `docs-sphinx` and `clean-docs-sphinx` targets
- travis tests the clean, html and pdf targets via the toplevel targets

Testing:
- travis should be green
- `ant docs-sphinx` should generate valid html and PDF docs, and these should match those generated with `make`

Follow on work could pass version information etc. directly from ant to sphinx to embed in the docs, and also generate documentation zips for the HTML and PDF docs.