Update API page in Piwik to show the Metadata report for all functions

[mattab]

Per title - we should highlight the Metadata output and favor its use as it is a much more useful dataset than standard API responses.

All these API functions that support Metadata would have a link "Metadata example". We could also clean up the existing list of links which is a bit long (XML, CSV, RSS Last 10 days, etc.)

[mattab]

Also, it would be VERY nice to link the API functions to the automated PHP DOC!

For example for UsersManager.addUser, we would link to: http://qa.piwik.org:8080/phpdocs/Piwik_UsersManager/Piwik_UsersManager_API.html#addUser

This would provide a nice help and avoid browsing PHP files to see the comments and parameter descriptions.

[mattab]

See also #1478 - I'm keen to make this page better, it's too geeky at the moment

[mattab]

When we do link functions to the PHP API generated doc, we should go through all functions and improve their documentations.

[mattab]

Critical to get the API doc to show the Metadata output more prominently.

[mattab]

Also, graphs export as PNG (fetched from metadata #2670) should be shown in the API export format list!