refs #1700 performance analytics

 * adding avg_time_generation to Actions.get + integration tests
 * adding sparkline for average generation time to Visitors > Overview
 * changing number formatting to 0.XXs instead of XXXms + test cases
 * tooltip for reports with avg. generation time: "average based on X hit(s)"
 * log import: support generation_time_milli (not only generation_time_micro)
 * example for importing generation time from logs in read me
Merge branch 'master' of github.com:piwik/piwik