Refactored CSVReader/first-class support for handling variable column rows

[vincentlaucsb]

• Refactored actual CSV parsing logic out of CSVReader
  • This logic is now located in internals::parse()
  • This refactor also allowed me to get rid of CSVGuesser and replace it with a more succinct function
• CSV parser now has first-class support files with variable columns instead of forcing users to override bad_row_handler() (which has been removed)
• Reorganized unit tests
• parse() now returns a CSVReader instead of a std::deque<CSVRow>