April 1 BIP16 switchover time is definite

Support for BIP16 has been approximately 70% of hashing power for the last two days, and is 54% over the last week, so we're going to announce that the BIP16 switchover time of April 1 is final and strongly encourage any pools/miners that haven't yet upgraded their software to do so in the next two weeks or risk hashing on a minority chain.

I'll be checking to make sure support doesn't slip before April 1.

I've learned a lot, and have started writing up a document that suggests how we can make the next upgrade process less painful for everybody; when I've got a reasonable draft I'll start a discussion about it here.

> Pretty graph of support over the last 100 blocks here:
>  github.com/bitcoin/bitcoin/

D'oh!  correct url for the pretty graph:
  http://blockchain.info/P2SH