fix issues #114, #129

this patch fixes issues when xml based epub are read. (openberg selects correct mimetype according to file extension)
I'm a bit nervous about sniffing, as we have no certainty that authors are not going to call their xhtml files ".html".

A (slower but more robust) alternative would be to try it with `application/xhtml+xml` by default, and if it fails, try again with `text/html`.