[urgent] dirty difficulty computation update

 - switching to a shorter averaging period.
 - should provide faster diff adjustments, while keeping *4 cap.
 - mandatory at block 101908 (chain should again be 'stucked' then,
   seeing current hashrate growth)
Merge branch 'dev-0.1.3' into release-0.1.3
Merge branch 'release-0.1.3'