Readd po w check

Please consider pulling these changes. I've read the foundation thread and added the following to this branch created from the master branch:

I've added some changes including re-adding the POW check @ block 30000
I've also changed the MinStakeAge to 12h as per the foundation threads vote numbers MaxStakeAge is 56d.
I've updated the version numbers.
I have left the the previous whitecoiners public key and the checkpoint for block 11100 rather than the 5800
I have not added supernode.
** I have built and tested this on Ubuntu 13.10 
** I have tested on the current block chain and everything is syncing ok.
** this now needs to be tested on Windows and OSX builds.

Cheers
Alcamie