added new function to pad barcodes

added new function to pad barcodes

-- expand_barcode
--   $barcode      source barcode
--   $prefix       prefix to add to barcode, NULL = add no prefix
--   $maxlen       maximum length of barcode; default to 14 if left NULL
--   $pad          padding string to apply to left of source barcode before adding
--                 prefix and suffix; set to NULL or '' if no padding is desired
--   $suffix       suffix to add to barcode, NULL = add no suffix
--
-- Returns a new string consisting of prefix concatenated with padded barcode and suffix.
-- If new barcode would be longer than $maxlen, the original barcode is returned instead.

Example usage:

UPDATE foo
SET barcode = migration_tools.expand_barcode(barcode, '21234', 14, '0', '');