Merge phantomjs page settings from Grunt config

Allow PhantomJS Page settings to be defined in grunt options and merged with webpage instance.
for example:
`options: {
  page: {
    settings: {
      webSecurityEnabled: false
    }
  }
}`

(in pr #62, if you wanted to change a [setting](http://phantomjs.org/api/webpage/property/settings.html) the whole settings-object would get overwritten and this broke things at least for me)