XHash implementaiton for Parallax

[andrepatta]

This is Parallax new proof-of-work algorithm. Please see the discussion that originated this change here: https://bitcointalk.org/index.php?topic=5560698.msg65873804#msg65873804

XHash is mostly Ethash but with a few changes to make sure existing Ethash ASICs and Ethash mining software aren't compatible with Parallax.

Ethash -> XHash changes include:

• Use the actual FNV-1 algorithm instead of the FNV-1 inspired used by original Ethash
• Use SHA3 256/512 instead of Keccak 256/512 used in original Ethash
• Epoch blocks from 30,000 to 720 blocks
• Target difficulty from 2^256 to 2^256-1
• Add Parallax magic PARALLAX\x01 to DAG seedHash

We'll publish a reference implementation for XHash as well as a GPU miner after mainnet launch.