Multi keychain support

Hi.

Thanks for creating this tools. It looks very clever and simple to me. We need to have more than one keychain in use because of different security levels in our company. I added this. Mainly I picked a notation of keychain:account. I think the best way to see the changes is to go into the updated readme.

In the beginning I started working on the cloned master, so the history might be weird. I'm still learning a lot and I'm not sure my code is as clean as yours, but this might be useful code to include in the main gem.

Let me know what you think. Cheers,

DPT