Blocks, Headers and Tx validation

[dakk]

#We need to integrate these validity tests for blocks, headers and txs:
https://en.bitcoin.it/wiki/Protocol_rules#.22tx.22_messages
https://en.bitcoin.it/wiki/Protocol_rules#.22block.22_messages

• Header validation
• Header validation - prev block
• Header validation - nbits pow
• Header validation - nbits and diff
• Header validation - median timestamp
• Header validation - future timestamp
• Header validation - checkpoints
• Block validation
• Block validation - Header validation (optional)
• Block validation - Txs > 0 validation
• Block validation - Coinbase validation
• Block validation - Merkle root
• Tx validation
• Loggable assert function (atm if the verification fail, it didn't display any hint about the problem)