New twoside mode.

[n . puttock]

LGTM.


http://codereview.appspot.com/144049/diff/1/2
File input/regression/paper-twoside.ly (right):

http://codereview.appspot.com/144049/diff/1/2#newcode15
Line 15: binding-offset = 5 \mm
I think this deserves a separate test, otherwise it just looks the same
as an ordinary page with slightly bigger but equal margins.

http://codereview.appspot.com/144049/diff/1/3
File lily/output-def.cc (right):

http://codereview.appspot.com/144049/diff/1/3#newcode173
Line 173: - left_margin_default
You'll have to add parentheses if you want to keep this indentation.

http://codereview.appspot.com/144049/diff/1/4
File ly/paper-defaults-init.ly (right):

http://codereview.appspot.com/144049/diff/1/4#newcode90
Line 90: twoside = ##f
I'm not too fond of this name, but can't think of anything better. :)

http://codereview.appspot.com/144049/diff/1/5
File scm/page.scm (right):

http://codereview.appspot.com/144049/diff/1/5#newcode320
Line 320: (ly:stencil-translate page-stencil (cons (prop 'right-margin)
0))))
You could simplify this a bit:

(if (ly:output-def-lookup layout 'twoside #f)
    (set! page-stencil
     (ly:stencil-translate page-stencil
      (cons (prop (if (even? number)
                      'left-margin
                      'right-margin))
            0)))

http://codereview.appspot.com/144049/diff/1/6
File scm/paper.scm (right):

http://codereview.appspot.com/144049/diff/1/6#newcode242
Line 242: (("foot-separation" . ,h) - '())
oops ;)

http://codereview.appspot.com/144049