Replace our po scanner with the one used by zotonic.

Fixes #23.

The example data given in issue #23 now parses OK:
```
1> po_scanner:parse_po_file("23.po").
[{header,"Project-Id-Version: Project\nReport-Msgid-Bugs-To: \nPOT-Creation-Date: 2012-02-10 17:50+0100\nPO-Revision-Date: 2012-02-10 17:50+0100\nLast-Translator: Thomas Schaaf <schaaf@komola.de>\nLanguage-Team: komola UG <team@komola.de>\nMIME-Version: 1.0\nContent-Type: text/plain; charset=UTF-8\nContent-Transfer-Encoding: 8bit\nX-Poedit-KeywordsList: _;gettext;gettext_noop;_t\nX-Poedit-Basepath: .\nX-Poedit-Language: German\nX-Poedit-Country: GERMANY\nX-Poedit-SourceCharset: utf-8\nX-Poedit-SearchPath-0: .\n"},
 {"PAGE_NOT_FOUND","Seite nicht gefunden!"}]
```