Property index

As a follow-up of https://github.com/openmicroscopy/openmicroscopy/pull/3020#issuecomment-55578777, this PR modifies the `indextemplate` for the custom `property` directive introduced by @aleksandra-tarkowska to reduce the size of the generated index. To test this PR, check the index page of the generated documentation (http://www.openmicroscopy.org/site/support/omero5.1-staging/genindex.html).

Additionally, the `property` directive is now documentation in the main README of the repository.

--no-rebase