Autodetect column types using the Excel file's cell types

[lcorbasson]

The rationale behind this commit is that if typing information is available in the Excel file, we should probably use it.

This is especially useful for Excel files with cells defined as text while seemingly containing numbers (phone numbers, numbers like "3,6" where one can't be sure of whether the comma is a decimal separator or not, etc.), which are currently distorted when imported.

[jpmckinney]

Thank you! I've added you to AUTHORS.rst.