add reverse method of extract(string)

during PO.parse, an extract(string) method is called on each string to
unescape some characters (like " and \). This process should be reverted
in the toString method.

The PO spec says, that all strings should be C-Strings. Otherwise tools
like msgmerge (from the gettext package) will fail parsing po files written
by this library.
add some real world examples for c-strings in po files

make sure, if a file containing c-string messages is identical after being
parsed and written afterwards.
Merge pull request #7 from Open-Xchange-Frontend/write_escaped

add reverse method of extract(string)