Main: Add chapter title lookup into transform()

Add code that checks for <title> tag and uses its content as a chapter title.
The lookup is done on the transformed html.
Main: Refactor epub generation

Rewrite generation of epub file from GEPUB::Builder to GEPUB::Book approach as
use of instance_eval in GEPUB code resulted into various issues with variable
scope.
Main: Implement collection and subtitle

Add class for recording optional information about book collection and add this
information into epub.
Main: Add documentation about gem