1 3 stable

This is a candidate pull for a new master for spree_print_invoice, update for Spree 1.3.

The changes to the gemspec are only suggested and should be reviewed prior to merging this commit.  It presumes the existence of a '1-2-stable' and '1-3-stable' branch, neither of which currently exist.  It also puts the Spree version at 1.3.1 since that's what I developed against, but I didn't know whether to list that version of 1.3.0.  I'll leave that decision to more knowledgeable minds.

There are two basic changes.  The admin buttons have been changed to an unordered list, so the partial that adds the invoice button(s) has been updated to create list items rather than bare links.

The second change is that the overrides which used to add the button have been consolidated into one and the name changed to match the new view that is being overridden.  One view (spree/layouts/admin) now handles the toolbar buttons for both the show and edit actions.