Implement a built-in concurrent CPU miner.

This commit implements a built-in concurrent CPU miner that can be enabled
with the combination of the --generate and --miningaddr options.  The
--blockminsize, --blockmaxsize, and --blockprioritysize configuration
options wich already existed prior to this commit control the block
template generation and hence affect blocks mined via the new CPU miner.

The following is a quick overview of the changes and design:

- Starting btcd with --generate and no addresses specified via
  --miningaddr will give an error and exit immediately
- Makes use of multiple worker goroutines which independently create block
  templates, solve them, and submit the solved blocks
- The default number of worker threads are based on the number of
  processor cores in the system and can be dynamically changed at
  run-time
- There is a separate speed monitor goroutine used to collate periodic
  updates from the workers to calculate overall hashing speed
- The current mining state, number of workers, and hashes per second can
  be queried
- Updated sample-btcd.conf file has been updated to include the coin
  generation (mining) settings
- Updated doc.go for the new command line options

In addition the old --getworkkey option is now deprecated in favor of the
new --miningaddr option.  This was changed for a few reasons:

- There is no reason to have a separate list of keys for getwork and CPU
  mining
- getwork is deprecated and will be going away in the future so that means
  the --getworkkey flag will also be going away
- Having the work 'key' in the option can be confused with wanting a
  private key while --miningaddr make it a little more clear it is an
  address that is required

Closes #137.

Reviewed by @jrick.