ensure TiddlyWiki documents are saved as UTF-8

previously special characters (ä, ö, ü, ß)  were being corrupted

this is a quick hack[1] to solve this particular issue with TiddlyWiki
documents; it should probably be encapsulated and applied to other parts
of the code (e.g. for RSS feeds), but I currently lack sufficient
understanding of the source structure

[1] based on http://stackoverflow.com/a/7807717