Fixed exceptions: now their messages are correctly printed

They weren't because I forgot to call the parent class constructor.
Added debug logging messages for constraints merge and pruning

Generates messages like:

versions.constraints: DEBUG: Merged constraints: <=2.0.0 and >=2.0.0
into ==2.0.0
versions.constraints: DEBUG: Removed constraint >=2.0.0 because it is
less restrictive than >2.0.0
versions.constraints: DEBUG: Removed constraint <2.0.0 because it is
less restrictive than <=1.0.0
Bug fixes: != constraints were not handled in merge() + detection of conflict between != and == constraints on the same version
Version change: 0.2.0