Prepare documentation for publication

[weierophinney]

This patch prepares the documentation for publication, and adds the build automation.

• Converts to mkdocs.
• Edits for content (fixes #9, #10, #11, #13).
• Adds tables lost during rst -> markdown conversion (fixees #12).
• Adds landing page.

Results are at https://zendframework.github.io/zend-permissions-acl/