[testnet] diff,adjustment: EMA retargetting.

 - Exponential Moving Average diff computation, over the last 2160
   blocks duration (roughly 3 days worth of blocks).
 - active on testnet since block 5182, not enabled on livenet yet.
 - testnet will eventually allow diff=1 mining, when last block was
   found more than 10 mins ago ; livenet will divide diff by two under
   same circumstances.
 - will probably be activated on livenet in a short future according to
   tests.
[core] target,adjustment: enabled ema-based target calculation.

 - retargetting now occurs at every block
 - new target calculated using exponential moving average block duration
   over last 2160 blocks (roughly last 3 days).
 - should allow quick exit from current stalled state by extra-reducing
   difficulty on longer blocks.
 - difficulty increments/cap reduced.
 - MANDATORY on block 101631
Merge branch 'dev-0.1.3' into release-0.1.3
Merge branch 'release-0.1.3'