Fixed OPF assertions. Added xprocspec test for epub3 validator.

* added xprocspec test support
* added xprocspec test for epub3 validator
* added epub3 test book that is supposed to be valid according to nordic guidelines
* fixed OPF assertions
    * dc:identifier and dc:title must be present (fixes #16)
    * removed test for `dc:date/@scheme` - scheme is not an allowed attribute on dc:date
fixed generic validation, test dtbook, impl. xml declaration test

* updated xproc-maven-plugin plugin, and commented it out since it doesn't currently work with xml catalogs
* changed charset from 'utf-8' to 'UTF-8' in schemas etc. The DTBook output still gets the encoding in lower case in the xml declaration though, due to the underlying implementation of p:store. The encoding attribute is case insensitive anyway, but at least in the meta elements we should probably try to enforce a convention.
* added reminder warning message to the epub3 validator about validating using epubcheck as well. epubcheck is still not implemented into pipeline 2.
* fixed issue with invalid DTBooks (generic validation); an error message instead of validation errors were shown.
* cleaned up html-to-dtbook.convert.xpl a bit, as well as fixed some issues
* added DTBook for testing (should be valid according to guidelines)
* updated test epub
* created packaged version of test epub
* implemented xml declaration assertion which lets xprocspec test whether a file contains the xml declaration or not