Added support for custom beginning_of_workday and end_of_workday

Hi, for some project I needed more advanced business times so I added "work_hours" configuration:

    BusinessTime::Config.work_hours = {
      :mon=>["9:00","17:00"],
      :fri=>["9:00","17:00"],
      :sat=>["10:00","15:00"]
    }

so you can specify working hours for each day also it supports 24h workdays. The real commit is this https://github.com/gacha/business_time/commit/b0d8e552201665062d5cbdb31b0f553a2fb19a00