import/export accounts

- remove account export functionality from CLI
- remove accountExport method,
- remove unencrypted-keys flag from everywhere
- improve documentation

output error message if unlock address is invalid (fixes the wierd "read /path: is a directory") msg

- accounts: remove Manager.getKey
- cli: for -unlock coinbase, use account manager Coinbase()

- extract accounts.getKey method - if given empty address it retrieves coinbase (first account)
- cli -unlock coinbase will unlock coinbase

- cli: add passwordfile flag
- cli: change unlock flag only takes account
- cli: with unlock you are prompted for password or use passfile with password flag
- cli: unlockAccount used in normal client start (run) and accountExport
- cli: getPassword used in accountCreate and accountImport
- accounts: Manager.Import, Manager.Export
- crypto: SaveECDSA (to complement LoadECDSA) to save to file
- crypto: NewKeyFromECDSA added (used in accountImport and New = generated constructor)

* Miners could stall because the worker wasn't aware the miner was done

c6d9629 added another test

git-subtree-dir: tests/files
git-subtree-split: c6d96293710a37489fa3b074a9fc228e0393f152

rlp: encode nil array pointers as empty list or string

XEth cleanup

bugfix with idle too long triggered after peer switch

* In case of uncle mining (e.g. same TS) the miner would stop if all
  threads happened to mine a potential uncle