improved conversion of lists

 * added class 'list-style-type-none' for lists with no bullets or numbers
 * improved conversion of 'li' elements from dtbook to html (fixes cases where block elements were created as siblings to inline elements)
 * figure element are now allowed inline (at least in some cases?)
 * added tests with lists containing figures, text and inline elements as siblings
Merge pull request #127 from nlbdev/improved-list-conversion

improved conversion of lists