Simplified origin filter

* use ActiveSupport's `.parameterize()` instead of own custom rules (e.g. umlaut-replacement)

**procedere**

1. call `.parameterize` on origin-string
2. prefix origin-string with '_' if first character is a digit ([turtle compliance][turtle-spec])

[turtle-spec]: http://www.w3.org/TeamSubmission/turtle/#sec-grammar