Extend AnonymizeIP to properly mask IPv6 addresses.

maskLength is now interpreted as "mask level" (0-4) and determines the number of bits to be masked.

Level	IPv4	IPv6
0	0	0
1	8	64
2	16	80
3	24	104
4	32	n/a

fixes #3710
Merge branch 'master' of https://github.com/piwik/piwik