extratorrent moves to extratorrent.cc
Split 'common/' into commit '2b0977bbbd7cb5a9ae3408f28b46d5a7b000d9d9'

git-subtree-dir: common
git-subtree-mainline: 79f3a9e9e17055bb243e4bac34f81321e67121ab
git-subtree-split: 2b0977bbbd7cb5a9ae3408f28b46d5a7b000d9d9
Added 2 important acquisition/retention metrics.