Fix foreign character display in receipts

Fix foreign character display in receipts

This commit makes two changes to the staff client print code to
help with the proper display of foreign characters:

1) Specify the utf-8 charset in the receipt Data URIs.
2) Properly encode the entire data string using encodeURIComponent
rather than encode().  Failing to do so was causing XULRunner to
be "creative" with the interpretation of the data.

Signed-off-by: Dan Wells <dbw2@calvin.edu>