sotorrent/so-posthistory-extractor: Updates to PostVersion, PostHistory, and test cases

Order post versions according to CreationDate instead of PostHistoryId:

• In the SOTorrent 2018-01-18 dataset, 283 posts created in 2008/2009 were not ordered chronologically (see "broken_entries" in "analysis_postversion_edit_timespan.R").
• Thus, we now order post versions according to their CreationDate (instead of using the PostHistoryId).
• Updated database schema and class PostVersion to include new member variable CreationDate.
• Update method PostVersionList.sort to use CreationDate.
• Add test case (post taken from "broken_entries", see above).
• Remove TODO (the described functionality can be achieved using the RootPostBlockId in select statements, after the database has been built)

Add test cases derived from lastest runs:

• add additional test case with empty post version
• inline stack snippet
• empty code block
• refactor parameter order when calling assertEquals

Minor changes to PostHistory such that test case for question 26365857 passes (inline stack snippets with language information)