Add supertest dev dependency
Add set(key, value, [domain]) method

Allows for setting context values at runtime, not just at init.
Similar to get(), it defaults to using the active domain.

Set a value like this:

    var domainContext = require("domain-context")
    domainContext.set('key', 'value')

Retrieve it later via a regular call to get():

    var domainContext = require("domain-context")
    value = domainContext.get('key')