Move Bio-Formats downloads page under the main repository

- Convert Bio-Formats downloads page into Sphinx format. Page can be generated
as usual with `make html` and sphinx_theme can be applied as for the regular
Bio-Formats documentation.
- Add release download page build as part of the Travis build
Fix indents in index.html
Merge pull request #681 from sbesson/downloads_page

Bio-Formats downloads page