Update documentation for ResultSets::CSV
Add documentation for ResultSets::Database
Update ResultSet registration to be more consistent with Comparator
Don't auto-open and auto-close during ScoreSet#each_pair

- ScoreSets::CSV#each_pair and ScoreSets::Database#each_pair now raise
  exceptions if they are not open for reading
- Matcher now calls ScoreSet#open_for_reading and ScoreSet#close
Add documentation for ScoreSet
Simplify constructors for CSV-related classes

- use :filename option instead of parameter in MatchSets::CSV and
  ScoreSets::CSV
- add :dir option to MatchSets::CSV and ScoreSets::CSV
- simplify passing options from ResultSets::CSV to its children sets
Change ScoreSets::Database and MatchSets::Database parameters

- use :database option in place of database parameter
- use sqlite database if no options are specified
- add :directory option
Add Helpers::CSV module to DRY up some common CSV stuff
Let ResultSet be usable by itself
Allow passing a score/match set directly into Configuration.new
Don't allow per-set options in ResultSets::CSV
Rename :database option to :conn in Database-related sets
Improve database-related sets

- add database helper module
- remove per-set customization in ResultSets::Database
- unify connection options across sets
add sum algorithm; implement weights for comparators
validate threshold
Update documentation for ScoreSets::CSV and MatchSets::CSV
Update documentation for ResultSet and Runner
Tweak method descriptions in Configuration
Tweak Dataset documentation
Fix ScoreSets::CSV documentation example