* cmd, consensus, eth: split ethash related config to it own
* eth, consensus: minor polish
* eth, consenus, console: compress pow testing config field to single one
* consensus, eth: document pow mode
`geth makedag <blocknumber> <path>` was creating DAGs for
`<blocknumber>/<epoch_length> + 1`, hence
it was impossible to create an epoch 0 DAG.
This fixes the calculations in `consensus/ethash/ethash.go` for
`MakeDataset` and `MakeCache`, and applies `gofmt`.
This commit adds pluggable consensus engines to go-ethereum. In short, it
introduces a generic consensus interface, and refactors the entire codebase to
use this interface.
