PO.Item
Fixes #5
I added a few more edge-cases to the tests for the new msgctxt field and revealed a bug during that. The default value has been set to empty string, but should have been null, since the spec says, this field is optional.
since the parser can extract this information, now, the writer should also be able to write it back to a po file
This will massively shrink the browserified footprint.
