Add bestblockhash and dirty version info to getpeerinfo rpc method

[sakdeniz]

It is very helpful to see the blockhash and dirty version number of other clients, especially when monitoring nodes in the testnet network.

[mxaddict]

@sakdeniz I think we already have blockheight and blocks synced, which you can add to get the current block height of the peer (so no need to get bestblockhash?)

About the dirty version of the node, I don't think that nodes broadcast that currently, so we might have to update the network code to broadcast the git hash or dirty version name.