Send number of contributors in ping data

See http://snippets.xwiki.org/xwiki/bin/view/Extension/Count%20contributors/

We should send several values:

• number of contributors1 (i.e. users who've edited pages in the past 1 year)
• number of contributors2 (i.e. users who've edited pages in the past 6 months)
• number of contributors3 (i.e. users who've edited pages in the past 3 months)
• number of contributors4 (i.e. users who've edited pages in the past 1 month)