Safe element append method
Switch of all LXML appends to new safe append method
Implement faster safe append

Fixes #58
Scan further back for bibliography lists
Undo unsafe optimization
Fixes to safe element addition method
Changes to size classifier
Better reference endmatter detection
More endmatter detection (avoid duplicate detection procedures)
Handle concurrent bibliographic entities missing author name.

Some reference styles do not repeat the author name and instead give "---." at the start of lines.

This causes us hassle when trying to work out the reference components. This commit changes all of these entries to be equal to the author above.
Documentation in reference linker
Reinstate disappearing headers

This will ensure that headers are never wrapped in blocks that prevent the XSL from treating them
Remove unnecessary print
Optional settings switches for list classifier items. #44
Option to adjust minimum size threshold in size classifier #44
Change all uses of value_for_tag to new non-static syntax
Large refactor of settings module
SizeClassifier aggression level made optional #44
Bibliography aggression level made optional #44
BibliographyClassifier aggression level made optional #44