Excel Reader should more gracefully handle datetime

[wjohnson]

Currently, if you have a date cell, openpyxl will return a datetime which is not json serializable by default.

I'm leaning toward converting it to unix epoch time and an int since Excel can force a date field into being a string with the tick symbol (`) but it depends on which one is more common: Storing the date as a string or storing the date as a int?