Allow correct parsing of multiline-string POs.

Bug: incorrect loading of multiline POs.
E.g.: 
```
msgid "Puede que te pidan "
"registrarte para ver la web."
msgstr "You may be requested"
"to register to view the site."
```
Some PO editors (like simplePO) generate these.

Fix: concatenate lines ending in double-quote with subsequent ones starting with double-quote before parsing.

Un saludo! ;)