add a blank line in the comments of the headers of the big.po file. This
breaks parsing the headers. msgcat of gettext tools just ignores empty
lines until a header is found and treats everything above as header
comment.
The change to the library itself fixes the tests again, after the blank
linke broke 3.
The gettext format specifies a msgctxt field for translated items. An
example of how to use this field has been added to the tests and the
parsing of this attribute has been implemented.