Improve the Visitors > Overview report #1297

• Add "Actions per visit" (Average value) sparkline and graph
• Change "Total time spent by the visitors" into "Average visit length"
• Graphs showing time should display it in "minutes" instead of seconds

These modifications should also apply to other similar pages:

• Referers > Evolution
• Visitors > Frequency & Loyalty

It could also process and report 'Unique page views' which are a better metric than Page views. This can be obtained by summing the nb_uniq_visitors for each page. For non day periods, we can use sum_daily_nb_uniq_visitors.

There would therefore be a new stats and sparkline below the graph.

see http://visualrevenue.com/blog/2010/06/choose-daily-unique-article-views-over-page-views.html

(In [2494]) Fixes #1297

• now reporting Actions per visit
• reporting Average Visit duration instead of total visit duration
• removed api getMaxActionsReturning() as it doesn't sound like a useful metric
• udpated tests to test the new metrics