fixed printing circular structures

* miner control moved to `admin.miner`
* miner option to set extra data
* block extra now bytes

@zelig we need to discuss this

@zelig look in to this please

@zelig this will need updating with the TD checking

* ChainManager allows cached future blocks for later processing
* BlockProcessor allows a 4 second window on future blocks
* Fixed tests

@zelig When blocks are in the future they'll be cached and processed at
a later time. Because of this the returned TD will be much lower than
the broadcasted TD.