Hukumusume: Add correct doctype and xhtml namespace

Add <xsl:output> tag into XSL file to force correct DOCTYPE for epub html pages.
Update the transformation to add xmlns attribute into <html> tag.
Hukumusume: Replace 'align=center' with css styling

The epub specification does not allow align attribude on <p> tag. Remove it
completely from output html and use style="..." for text horizontal align.
Main: Split encoding into input and output encoding

The output encoding of HTML document may need to be different from input
encoding. Unless specified by --enc-out argument, output encoding is same as
input encoding.

For example, when creating simple HTML pages for ebook reader, we may
need to convert from UTF-8 to Shift_JIS. Or when from Shift_JIS to UTF-8 when
creating epub.
Main: Change output from html to xhtml
Main: Add class for collecting info about URL

Add PageInfo class that gathers information about all processed URL so that it
can be used in ebook creation later on.

Specifically, we collect file name, names of all media files and title of the
web page that will serve as chapter heading.
Main: Add class for creating epub

Add class Book that loads YAML file with ebook metadata and then uses
GEPUB::Builder to create epub book from both metadata and downloaded web pages.
Main: Application script uses Book class to create epub

Add command line option for supplying epub metadata. Add code for passing
PageInfo instances into class for epub creation.
Utils: Fix xpath test tool and remove obsolete file