based on http://adeduke.com/2015/08/how-to-create-a-private-ethereum-chain/

customGenesis.json

{
	"nonce": "0xdeadbeefdeadbeef",
	"timestamp": "0x0",
	"parentHash": "0x0000000000000000000000000000000000000000000000000000000000000000",
	"extraData": "0x0",
	"gasLimit": "0x8000000",
	"difficulty": "0x400",
	"mixhash": "0x0000000000000000000000000000000000000000000000000000000000000000",
	"coinbase": "0x3333333333333333333333333333333333333333",
	"alloc": {
	}
}