[patch:lib] Remove key-word arguments from Paper#id and Paper#url

[eonu]

• Remove key-word arguments from Paper#id and Paper#url.

  Previously, Paper#id and Paper#url accepted a version key-word argument, which was a boolean variable indicating whether or not to include the version number in the ID or URL.

  This has now been changed to a regular argument, which still defaults to false.

  paper = Arx.get('cond-mat/9609089')
  
  # Old (no longer works)
  paper.id(version: true)
  paper.url(version: true)
  
  # New
  paper.id(true) #=> "cond-mat/9609089v1"
  paper.url(true) #=> "http://arxiv.org/abs/cond-mat/9609089v1"