Use @ instead of this in Bookmark class
Fix issue #579 - support for DAISY pipeline books

This commit introduces some major changes to the internal class
strucures. These changes include:
 - Merging the former Playlist class into both the Player and the Book
 - Refactor the SMILDocument so that it is decoupled from the Section
 - Section searching/traversing is now located in the Book
 - NCC sections that link to the same SMIL file now work, by also
   sending the proper fragment
 - Since sections are actually "bookmarks" and not container elements
   for segments, it's no longer directly possible to access a section
   from a segment, without calling a utility method on the Book

In general we're trying to seperate responsibilities, so that the Book
has no clue as to were the Player currently is and is only used as a
data model.