fix issues #114, #129

this patch fixes issues when xml based epub are read. (openberg selects correct mimetype according to file extension)
About regex : yes, that's it. In fact, I tried to "follow" OPS specifications here :

http://www.idpf.org/epub/20/spec/OPS_2.0.1_draft.htm

We can read this :

    Reading Systems must be XML processors as defined in XML 1.0 

So, following this way, I defined default mimetype to xml. We don't any reference to "text/html" mime type in the spec.

About sniffing : First, I wanted to use the mapping between file name and media-type attribute in the content.opf file. But, if we have a look at "le-petit-prince" ebook, the media-type defined for "index.html" is not "text/html" but "application/xhtml+xml" ! And the xml parser fails because index.html file is not xml compliant...

So, I think that the only way to make it working is to add this workaround (based on file extension). ok to add comment to explain that workaround.