openpaas-gatling#19: Provided a way to load the main.js file

[itswillta]

Resolves https://github.com/OpenPaaS-Suite/openpaas-gatling/issues/19.

Example usage:

  def openCalendarSPA(): ChainBuilder = {
    group("openCalendarSPA") {
      exec(LoginSteps.loadLoginTemplates)
        .exec(LoginSteps.login())
        .exec(StaticAssetsSteps.loadIndexHtmlAndMainJs(CalendarSpaName)) // CalendarSpaName = "calendar"
       //...
    }
  }