Make Diff a class (and not a module)
Add ox dependency
Delete diff_spec in wrong location
Fix spec.summary in gemspec
Add parsing of treebank xml files
Add report method for sentence
Fast attempt at a comparison

Needs refactoring of course
Refactor to make use of a Container module
Delete obsolete method

counter_hash is in the Module HashContainable
Allow HashContainable#new without args
Add a DiffContainer object instead of a plain hash
Delete obsolete debugging method
Chaotic refactor to replace hashes with objects
And even more indirection through objects
First xml markup of Difference class
Get rid of as_xml divergence in Difference
Update to_xml logic
Fix error in container_to_xml
Rename diff container classes
Get rid of diff inst var in Sentence