change coinbase maturity time

This increases the time before mined coins are allowed to be spent from 30 minutes to 2 days in order to better protect merchants (and everybody else) in case of a major fork. With this change, coin loss should be limited to those mining the "wrong" fork as long as you don't stay on the fork for more than two days (except for targeted attacks made possible by a major fork). For a discussion of this change see #315

The change is targeted to start at Block 160,000. That should be updated to match our fork target for the difficulty algorithm. Testing is always welcome.