Add missing properties for key stats API

[bingxie]

Address to issue: #49

Add missing properties for key stats API:

Missing properties list:

      property 'employees'
      property 'avg_10_volume', from: 'avg10Volume'
      property 'avg_30_volume', from: 'avg30Volume'
      property 'ttm_dividend_rate', from: 'ttmDividendRate'
      property 'max_change_percent', from: 'maxChangePercent'
      property 'day_30_change_percent', from: 'day30ChangePercent'
      property 'day_30_change_percent_s', from: 'day30ChangePercent', with: ->(v) { Resource.float_to_percentage(v) }
      property 'next_dividend_date', from: 'nextDividendDate'
      property 'next_earnings_date', from: 'nextEarningsDate'
      property 'pe_ratio', from: 'peRatio'
      property 'beta'