Allow grid redefinition #663

Squashed commit of the following:

commit 48e95b849f66da325f933564a687b5fe5711c4d3
Author: Gregory Brown <gregory.t.brown@gmail.com>
Date:   Sun Feb 16 09:24:22 2014 -0500

    Add some documentation about new Grid behavior

commit 420bee993f02a72eb23543f0743f09c081056e83
Author: Simon T. Ravn <cs2@cs2.dk>
Date:   Fri Feb 14 16:07:37 2014 +0000

    More logical way of resetting boxes

commit 9ff499a1ee234e7f48c6495161f09af82ced23fc
Author: Simon T. Ravn <cs2@cs2.dk>
Date:   Sat Feb 8 16:18:16 2014 +0000

    Always reset boxes to allow for redefinition of grid
Merge branch 'master' of github.com:prawnpdf/prawn