Redefined most XML parsing functions,more helpers

This commit as compared to previous ones has much more changes as:

> The xml parsing functions were not generic enough to my needs and needed to be redefined as well as renamed .
> I also set up the document tree for the document using these parsers.All that is now left is to serve these to a webview.
> Fixed a typo in epub-document.h

TODO: integrate with Atril frontend.