add parsing support for msgctxt field of poitems

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.
implement writing msgctxt field

since the parser can extract this information, now, the writer should
also be able to write it back to a po file
add distfiles

as compiled by the grunt task. These come in an own commit, since I
didn’t want them to clutter the actual implementation commits.
add documentation for the new field

possibly makes developers happy to have this field documented
Merge pull request #3 from Open-Xchange-Frontend/support_msgctxt

Support msgctxt field of items