This avoids having to query the coinbase when creating the miner, which
in turn eliminates the dreaded startup error when no accounts are set
up. Later, this will also allow us to simply restart the miner when the
user picks a different coinbase.
This causes a lot of changes in other packages. These are included in
this commit because they're impossible to separate.
The ethereum command line interface is now structured using subcommands.
These separate the different tasks it can perform.
Almost all flag names are backwards compatible.
The key tasks have not been ported to subcommands since they will be
replaced by the new accounts infrastructure very soon.
