Add set(key, value, [domain])

This PR adds a set() method to domain-context. It allows setting context values at runtime.

For example you could do something like this:

```coffeescript
domainContext = require("domain-context")
app = express()
# ...
app.use domainContext.middleware()
app.user (req, res, next) ->
  contextReq =
    id: req.ip
    url: req.url
  domainContext.set "req", contextReq
```

Also, I added supertest as a dev dependency.