Add coinbase extras

[stringhandler]

Add the ability to add a coinbase extra to the coinbases. Each miner can supply their coinbase extra and broadcast it as part of the share.

Squad data

The squad data must not be included in the coinbase extra when broadcasting shares, but when the local p2pool gets the template, it should add the squad to it's own coinbase using the following format:

1 byte 0xFF to say we are using Type-Length-Value
1 byte: 0x02 Type marker for squad
1 byte: The length of the squad-id, n
n bytes: The squad id

To accommodate other custom messages, like the coinbase the miner would have included if it were solo mined, we should also append this at the end.

1 byte: 0x01 Type marker for custom message
1 byte: length of custom message m
m bytes: custom message.