Ebookmaker

- fixed an ancient bug in EPUB2 pageno handling. Having two children ids in a pageno-class element no longer generates validation errors for EPUB2 files. Yes, it seems odd that a book would have two page anchors in one page number floating element, but it makes sense if you look at the rendered HTML (501), and it's no reason to mock people.

- bad things happen if there's text at the top level. often this is a result of bad html. pre_parse now takes care of this

There remain problems converting HTML 4.0 files.
- fix failed txt build when boilerplate is not found
- fix failed txt build when boilerplate marker appears twice

- fixed bug in colgroup wrapping
- Ebookmaker will now NEVER break a page in the middle of a table, a list, or a figure.
- PG boilerplate is inserted in EPUB2 files as well
- Fixed issue with special characters in the boilerplate dividers causing txt builds to fail
- recognize previously marked pg-header, etc., such as from rst

- fixed a mismatch between the classname given to the cover and the corresponding css
- added CSS page breaks before footer and after header
- BeautifulSoup doesn't convert entities in script or style elements because HTML5 specifies these as CDATA. So our code has to handle cases where there are unexpected entities there.
- remove ALL non-default attributes in replaced img, not just alt

- xml escape headers extracted from text files