heading towards a solid v1.0

This is the direction I'm heading:

https://github.com/coolaj86/node-nat-upnp/blob/v1.0/lib/nat-upnp/ssdp.js
- switched from shimmed EventEmitter promise-ish things to proper Promises using `global.Promise || require('bluebird').Promise`
- pulled the async code out of the constructor (since you can't pass callbacks or return promises from a constructor) and moved it to an `init()`
- removed mutual dependencies

I plan to switch it to return however many IGDs respond.

Start by comparing the current implementation with lib/nat-upnp/ssdp.js at the linked v1.0 revision, focusing on the Promise handling, constructor versus init() flow, and mutual dependencies. Done means those changes are reflected and the implementation returns every responding IGD as described in the issue.

Written by the indexing model from the issue text.