Merge default options with config files if property not specified

At present, most mosca config file options do not specify a logger object.
That makes them break when, for example, trying to use verbose output mode.

Example `mosca.conf`:

    module.exports = {
      port: 1883,
      backend: {
        type: "redis"
      }
    };

Then run:

    mosca -c mosca.conf -v

Will dump an error like:

    $ mosca c ./mosca.conf -v
    
    /usr/lib/node_modules/mosca/lib/cli.js:96
          opts.logger.level = 30;
                            ^
    TypeError: Cannot set property 'level' of undefined
        at /usr/lib/node_modules/mosca/lib/cli.js:96:25
        at cli (/usr/lib/node_modules/mosca/lib/cli.js:232:12)
        at Object.<anonymous> (/usr/lib/node_modules/mosca/bin/mosca:3:22)
        at Module._compile (module.js:456:26)
        at Object.Module._extensions..js (module.js:474:10)
        at Module.load (module.js:356:32)
        at Function.Module._load (module.js:312:12)
        at Function.Module.runMain (module.js:497:10)
        at startup (node.js:119:16)
        at node.js:901:3

This patch makes sure all "default" options are set to default values (merging default options with those specified in a config file) instead of outright replacing them, allowing configuration files to override only what is specified, while having a reasonable guarantee that the rest of the code which may depend on a default value can continue to do so.  It also has the side-effect of making the current examples work better.