Implemented double page support.

Double page view will be visible on landscape orientation.

You can toggle various double page options via the following...
- ReaderMainToolbar doublePageButton.
- ReaderConstant defines:
READER_ENABLE_DOUBLE_PAGE
READER_ENABLE_DOUBLE_PAGE_CONSISTENT_ZOOM
READER_ENABLE_DOUBLE_PAGE_FIRST_PAGE
- ReaderViewController properties:
BOOL doublePage
BOOL doublePageConsistentZoom
BOOL doublePageFirstPage