This change introduces 

    ethkey changepassphrase <keyfile>

to change the passphrase of a key file.

This commit:

- Adds a --msgfile option to read the message to sign from a file
  instead of command line argument.
- Adds a unit test for signing subcommands.
- Removes some weird whitespace in the code.

ethkey is a new tool that serves as a command line interface to
the basic key management functionalities of geth. It currently
supports:
 
 - generating keyfiles
 - inspecting keyfiles (print public and private key)
 - signing messages
 - verifying signed messages