Calculate number of returning unique visitors

[zawadzinski]

references #2646

add option to config to enable processing returning unique visitors for week/month/year/range periods (as for normal unique visitors)

implement processing thme in archivePeriod() function in VisitFrequency plugin

[mattab]

The data is now found in the API response, eg:

http://demo.piwik.org/index.php?module=API&method=API.get&format=XML&idSite=7&period=week&date=2013-10-07,2014-04-06&expanded=1&token_auth=anonymous&filter_limit=26

contains:

<nb_uniq_visitors_returning>1069</nb_uniq_visitors_returning>

We do not show it in the UI but more will be done in #539 so please comment there with feedback on how to make the Engagement report better (as we can do a lot to improve reports in general!).