Update for hard fork at block 25200.

Change difficulty algorithm to re-evaluate every 48 blocks (2 hours)
from block 25200 onwards. The difficulty will also go down at a faster
rate than it goes up in the event it is too high.

Add more checkpoints only using blocks which have transactions in them.
This will further cement the correct blockchain.
Merge pull request #1 from Petr1fied/master

Update for hard fork at block 25200