New twoside mode.

New twoside mode.

* This patch allows to specify different margins for odd and even
    pages by setting two-sided to true in the \paper block
  * The corresponding settings are inner- and outer-margin, there
    exists the possibility to set a binding-offset, too
  * These values are stored internally as left- and right-margin,
    though. Translation is done in page.scm:make-page-stencil
  * Small code cleanup in paper.scm:set-paper-dimensions