Ref #59: Display Unicode characters available in the used font for text/html

Unicode support is not supported for `displayImagePage:`, `displayPlainTextPage:` and `processDocument:` in Scamper and not test exists yet.

In addition, the font have to support the characters - otherwise Scamper will display a question mark.

There is still a problem concerning HTTP GET for inputs, if another encoding is requested for the URL #92.