Merge branch 'update_bitcoin_secp256k1_lib2' of https://github.com/Gustav-Simonsson/go-ethereum into Gustav-Simonsson-update_bitcoin_secp256k1_lib2

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