Fixes #2742, added support for getting subtable reports through metadata API and added necessary metadata so existing subtable reports can be obtained.

Notes:

  * Added ability to test subtable API actions.
  * Added subtable metadata to methods in Actions, CustomVariables & Referers APIs.


git-svn-id: http://dev.piwik.org/svn/trunk@6800 59fd770c-687e-43c8-a1e3-f5a4ff64c105