add json configuration option for tests

  * provide a cli argument for config.json file
  * parse arguments & options from config.json
  * ensure config.json is valid and can be required
  * ensure args & opts are stored back in phantom.casperArgs.raw
  * fixes #745

  - a config.json can look like:
    {
      "includes": [
        "inc1.js",
        "inc2.js"
      ],
      "log-level": "debug",
      "paths": [
        "test.js",
        "sub1"
      ],
      "post": [
        "post1.js",
        "post2.js"
      ],
      "pre": [
        "pre1.js",
        "pre2.js"
      ],
      "verbose": true,
      "xunit": "results.xml"
    }

  - tests can be run using:
    $ casperjs test ./config.json

  - note that, the config file is not a key word argument, since
    there are no additional arguments that you need to pass into
    the cli, if you provide a config.json.