When providing no passphrase, don't encrypt the key.

[abourget]

This was annoying as it has a different behavior than OpenSSL. If you added no password, the key was still encrypted, which made streamlining certain processes much harder.

Would fix #44

[yichengq]

I like this PR and would like to merge it to fix 44.
But i don't want to merge it in an half state.
i play with it a little, and find that init subcommand also prints out the line that Enter passphrase (empty for no passphrase):, and it does encryption. And export --insecure when passphrase is empty will fatal.

[abourget]

Oh, I see .. I'll see when I have time to wrap it up.

[bkleef]

+1