Key import and export

Key import and export

Introduces two new RPC calls:
* dumpprivkey: retrieve the private key corresponding to an address
* importprivkey: add a private key to your wallet

The private key format is analoguous to the address format. It is
a 51-character base58-encoded string, that includes a version number
and a checksum.

Includes patch by mhanne:
* add optional account parameter for importprivkey, if omitted use default