Set slide design per slide

[raphink]

This patch adds a new slide-design directive which allows to set the slide design used in the following slides, for example:

A first slide using default design
-----------------------------------

Some content

.. slide-design:: New Design

Another slide using the "New Design" design
--------------------------------------------

Note that this PR also includes https://github.com/mattharrison/rst2odp/pull/20

[raphink]

I updated the code with your remarks.

[raphink]

Here is an example of rst document using this functionality: https://github.com/raphink/augeas-talks/blob/master/rmll-2012/rst/augeas-rmll-2012.rst