Change the extension of the documentation files from .txt to .md so that they can processed as markdown and displayed in github.

(being respectful with occurences that might change the behaviour, i.e. default database prefixes)

Instead of always adding the endpoint to the metadata, require
setting a separate option to enable it.

git-svn-id: https://simplesamlphp.googlecode.com/svn/trunk@3194 44740490-163a-0410-bde0-09ae8108e29a