Mac base date is not used correctly
*Created originally on Bitbucket by [bjwebb (Ben Webb)](https://bitbucket.org/%7B6fc79bf2-665b-41ff-898d-b74e9065badb%7D/)*
Openpyxl reads the property from the workbook, and correctly propagates this information to the `base_date` attribute on a cell. However, the actual `value` attribute is wrong.
e.g. using https://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/663589/GGIS_Grant_Awards_2016_to_2017_2017-10-27_1621.xlsx
```python
>>> import openpyxl
>>> wb = openpyxl.load_workbook('/home/bjwebb/Downloads/GGIS_Grant_Awards_2016_to_2017_2017-10-27_1621.xlsx')
/home/bjwebb/opendataservices/cove/.ve/lib/python3.5/site-packages/openpyxl/reader/worksheet.py:318: UserWarning: Unknown extension is not supported and will be removed
warn(msg)
>>> wb.excel_base_date
2416480.5
>>> from openpyxl.utils.datetime import CALENDAR_MAC_1904
>>> CALENDAR_MAC_1904
2416480.5
>>> from openpyxl.utils.datetime import CALENDAR_WINDOWS_1900
>>> CALENDAR_WINDOWS_1900
2415018.5
>>> wb['GGIS_Grant_Awards_2016_to_2017 ']['F2'].value
datetime.datetime(2012, 10, 2, 0, 0)
>>> # We expected datetime.datetime(2016, 10, 3, 0, 0)
>>> wb['GGIS_Grant_Awards_2016_to_2017 ']['F2'].base_date
2416480.5
>>> wb['GGIS_Grant_Awards_2016_to_2017 ']['F2'].base_date is CALENDAR_MAC_1904
True
```
I notice that the [from_excel](https://bitbucket.org/openpyxl/openpyxl/src/5983d4ba5c18b85171185e8b1ca136876ec52864/openpyxl/utils/datetime.py#lines-84) method appears to do what I want, but this is called without an offset in [openpyxl.reader.worksheet](https://bitbucket.org/openpyxl/openpyxl/src/5983d4ba5c18b85171185e8b1ca136876ec52864/openpyxl/reader/worksheet.py#lines-225), and is commented out in [openpyxl.cell.cell](https://bitbucket.org/openpyxl/openpyxl/src/5983d4ba5c18b85171185e8b1ca136876ec52864/openpyxl/cell/cell.py#lines-293).
issue