Allow comma-delimited list of UUNs

When bulk-creating accounts, allow a comma-delimited list of UUNs (in
addition to a whitespace- or new-line-delimited list of UUNs)
In _create_user() change default error

In _create_user() change the default WP_Error message from an empty
message, to “Cannot find user's information”
Add notes on bulk-creating new accounts

Add some notes to the ReadMe about how to use the EASE plugin’s
bulk-account-creation tool (in create.php)