Initialize SentenceDiff with a sentence

... instead of the plain id to be able to hold the total size of
sentences and the id. Useful for the upcoming report.
Extract Report class

Report is the new superclass of Gold - counts totals of all interesting
stuff, while gold uses these totals to count differences.
Distinct private and public methods in Diff
Refactor of Diff for more flexibility

The goal is to add Reports on their own.
Add report method to Diff
Add big unit test to handle Diff#report
Call report on each report in Diff#report
Fixes HashContainable#merge_reports

Might need another fix, next tests will show.
Improve HashContainable#merge_reports
Take multiple reports in merge_reports
Unfocus
Call comparator#to_s in Sentence#compare

... so that we can hide more complex implementations behind (as needed
with postags)
Start more complex Postag interpretation
Add Postag#clean_analysis

... to return only datapoints that actually have data.
Enable Postag#report
Fix erroneous postag setter in Word
Enable Sentence#report
Add HashContainable#hash_to_xml
Enable Report#to_xml
Hack tagnames in HashContainable

Should actually use proper xml classes such as

<lemmata total="122">
  <lemma name="sum1" total="2"/>
</lemmata>