[public_key] add public_identity_file option
[public_key] bump version to 0.7.2
[public_key] fail if neither password or public key is set
[public_key] read pubic key from file before passing to fog

 - handle passwords better
 - fail if no password given and either public or private key is not set
[public_key] update README
[public_key] bump version to 0.8.0
Merge pull request #19 from sandfish8/public_key

be able to set both private key (identity_file) and public key (public_identity_file)