Add configuration options

* Added creation_date and creation_time as configuration options. These are necessary if you're trying to re-create a file exactly as it was originally generated. Defaults are current date/time.
* Added originating_dfi as a configuration option. While this is typically the first 8 positions of the immediate origin, many banks will have it be a specific value that sometimes has internal meaning to them. Users of this module need to be able to specify this value, but the default remains the first 8 positions of immediate origin.
* Setting the batch header record "Company Descriptive Date" field to EFFECTIVE_DATE is correct. NACHA recommends, but does not require, that RDFIs print this value on the receiver's statement. The effective date, not the current date, would be the correct value to show on a bank statement.