Add support for SVN authentication (mod_authz_svn)

Add dependency for authz_svn. This is needed, since the mod is included in the dav_svn system package. Thanks @hunner for pointing this out!

- Add parameter for controling the enabl-/disabling of the authz_svn module. (Disabled by default)
- Optimize the dependency management

Mark dav_svn module as a module which has optional setings.

Add loadfile_name parameter for better module load order

Add acceptance test for dav_svn module