Specify 'soap' as a peer dependency, and update to use soap 0.2.x

I'm using your package - thanks for sharing.

I've added a couple of suggested changes:

I found that the 'soap' module had changed its syntax: `create` has become `createClient`.
I also found how to specify a particular version of the soap library be installed, so that `npm install` "just works" and you don't need to remember to `npm install soap`.