Add QueryPair object, improve __repr__

[alecraso]

Changelist

• QueryPair class created
  • Adds a layer of abstraction on the queries and databases that comparisons are run against
  • In particular, this allows for the "right" query to directly reference the output of the "left" query
• ComparatorSet.from_list method removed
  • The logic was inherently complex and repetitive, and the from_dict method gives identical functionality that is much more explicit
• All __repr__ strings improved to better follow convention and be generally more readable
• DbConfig logging changed to DEBUG level
• QueryResultCol class created
  • Improves abstraction on column of data from the QueryResult
  • Avoids iterating over entire result to just pull one key from each row
  • Gracefully handles unicode formatting issues in python 2
• Adds tests to bring coverage back to 100%