Logs are now recorded per transactions instead of tossing them out after
each transaction. This should also fix an issue with
`eth_getFilterLogs` (#629) Also now implemented are the `transactionHash,
blockHash, transactionIndex, logIndex` on logs. Closes #654.

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

- add validation on TxMsg checking for nil
- add test for nil transaction
- add test for zero value transaction (no extra validation needed)

- rename Validate -> ValidateFields not to confure consensus block validation
- add nil transaction and nil uncle header validation
- remove bigint field checks: rlp already decodes *big.Int to big.NewInt(0)
- add test for nil header, nil transaction

- add validate method to types.Block
- validate after Decode -> error
- add tests for NewBlockMsg

There is no reason to keep them as ethutil.Bytes.

* Add from to eth_transact / xeth.Transact and add static pass in lieu
  of integrating with native Mist window for user passphrase entry
* Make eth_accounts return AccountManager.Accounts()
* Add a Generate Key menu item in Mist