Added: Support for Chapter sub levels.

Rev. 2.53 - 2013-10-05
* Added: Support for Chapter levels.
* Added functions:
*  ->subLevel() to indent one level under the current, additional
chapters are added under that.
*  ->backLevel() to step one level back to the parent of the current
level.
*  ->rootLevel() to step back to the root of the navMap.
*  ->getCurrentLevel() to get the current level indentation (root = 1).
*  ->setCurrentLevel(int) to set the current level indentation (1 or
less returns to the root, same as ->rootLevel()).
* ->buildTOC now reflects this level indentation if present.
* ePub250 is otherwise compatible to the previous version, and no
modifications are needed if you don't need this feature.