Add options for overriding methods in broker creation

[pkinney]

authenticate, authorizePublish, authorizeSubscribe, and published methods can be passed in the configuration options when initializing the broker or can be set after the fact as it works now:

var aedes = require('aedes')({
  authenticate: function (client, username, password, callback) {
    callback(null, username === 'skroob' && password === '12345')
  }
)};

Also refactored the options ab it to use a defaults function for clarity.

This addresses #9

[mcollina]

Thanks, releasing soon!