webdriver-manager Selenium hub option and direct binary download URLs

This pull request adds a --hub option to webdriver-manager. This option runs Selenium Grid as a hub role, useful for running multiple browser tests at once, or using PhantomJS.

It also adds the ability to specify direct download URLs for webdriver binaries (selenium-standalone-server, chrome, iedriver) - this is useful to overcome specific bugs that have fixes that are not yet in the releases, but can be fixed by using a private build of the tip.

Direct download urls are optional, and specified in `package.json`, like so:

    "webdriverVersions": {
        "selenium": "2.43-httpclient-fix",
        "seleniumDownloadUrl": "https://github.com/adamfeuer/selenium/releases/download/selenium-standalone-2.43-httpclient-fix/selenium-server-standalone-2.43-httpclient-fix.jar",
        "chromedriver": "2.10-fix-a",
        "chromedriverDownloadUrl": "https://github.com/adamfeuer/path/to/some/directory/chromedriver-2.10-fix-a.zip",
        "iedriver": "2.42.0-fix-b" ,
        "iedriverDownloadUrl": "https://github.com/adamfeuer/path/to/some/directory/iedriver-2.42.0-fix-b.zip"
    }
style nit (here and below): use single quotes