Pyramid 1.5a2 compatible

I've pinned Pyramid to 1.5a2 in 7d8e025 in order to be able to use `add_default_renderers` method committed in 53d0f85.

But I'm not sure if we should force this migration to Pyramid 1.5 on current version of Kotti. If so we could remove `add_default_renderers` call and import `DEFAULT_RENDERERS` from `pyramid.config,rendering` instead.

Notice that there are still some methods that are deprecated in Pyramid 1.5 that we will need to look at.