Add code to allow white space in a weekday mask:

"Sun Mon Tue"

Also add additional test cases to the unit tests.

The previous checkin has examples for working with datetime64 objects and
the busdays functions.  Edit the docstrings for busdays to harmonize them
with the language used in the examples.

Instead of saying "business days", now say "valid days".  The default
valid days are Monday through Friday, business days, but other sets of
valid days are possible.