Allow tags to have special characters

[sdcosta]

People should be allowed to tag in whatever language they like.

Link to relevant discussion http://sinless.org/pipermail/gregarius-dev/2005-August/thread.html#310

Now that we have a function like sanitize_title maybe we could also sanitize the tags.

[mdodoo]

This ticket is a duplicate of #38. However, I think that this ticket provides what Marco would call a "compelling use case" for adding more characters to the tagging system.

[sdcosta]

This will only get fixed after #376 is fixed.

[mbonetti]

should be fine as well.