Calculate maximum threat of each type

Get rid of lsb() in threat calculation by calculating maximum threat of
each type
Rewrite previous patch with cleaner code

Non functional change
Multithreat

We give a weight of 1 to the maximal threat, 1/4 to the second threat,
1/16 to the third, etc.
Calculate maximum threat for hanging pieces

Use the max_threat() helper function to estimate more precisely the
best hanging piece threat.
SPSA tuning for Threat array

SPSA tuning for Threat array
Use tuned values for threats

Use tuned values for threats
Fix spaces in code

Light editing for spaces and comment in the code.
Merge pull request #2 from snicolet/threats

Threats