fix for auroracoin 1.2.0.0 with hardfork at block 5400

modifies BLOCK_PERIOD. Dirty hack in data.py, because blockperiod
normally isn't variable. To be removed some time after block 5400.
Merge pull request #2 from mmouse-/block5400

fix for auroracoin 1.2.0.0 with hardfork at block 5400