PGN library

[bahmanm]

Create bPGN, a PGN library in a way that it can be used by bCS itself and others if they want to.
Unlike the current implementation (PGN.groovy) which is stateful (i.e DB) and tries to mimic a potential model layout of bCS, it should be stateless and only try to fairly represent a PGN structure.

For example:

class Tag {
  String key
  String value
}

class Ply {
  String san
  Ply next
  Ply prev
  List<Ply> variations

  String commentBefore
  String commentAfter

  String nag
}

class Game {
  List<Tag> tags
  Integer startngMoveNumber
  Ply first
}