Signature handling

[ebuchman]

Params file should contain fields to specify the signers, including their name and title and which counter party they relate to. The vendor name is already in the params, but the client name is not.

Down the road, client signing info should be a global configuration value in claw so you don't need to type it in every params file, but just in some global config file. We'll also want to support using claw from multiple entities on one machine, so will need a way to easily switch the global configuration.