2.0.0: Command options

The second major release adds the possibility to pass additional options to all commands, including read preference as well as read and write concerns.

• Merged PR #40: Implement read and write concerns, implementing #11.
  See https://www.mongodb.com/docs/manual/reference/read-concern/ and
  https://www.mongodb.com/docs/manual/reference/write-concern/
  (@thekid)
• Merged PR #41: Remove deprecated Collection::command(), superseded in
  1.4.0 with the new run() method, see #21.
  (@thekid)
• Merged PR #39: Refactor all methods to receive options varargs. Passing
  additional options to methods such as find can be accomplished by
  creating Options instances.
  (@thekid)