Adds french Tinymce locale.
Fixes updating of page urlname path.

The logic of the Page#set_urlname method does not take the root page into account. So if one updates a language root page, then the urlname would include the root page urlname ('root').

This commit also includes an addition to the page info view. It now includes the complete urlname path.
Allow Kaminari version greater then 0.15

Fixes #617
Fixes essence_name in essence generator

Essence name was all snakecase, but we need it underscored for template names and css class names.
Add a definition in page_layouts to render pages with certain layouts.

For pages, we have a new PERMITTED_ATTRIBUTE 'render_layout', which
allows us to define a layout to render the page with. Our use case for
this is to render certain pages with a layout that's not the
'application' layout.

In the pages controller, we check to see if the page has a defined
render_layout. If so, we get the render_layout, and call render with the
render_layout. If not, we just do whatever we did before.

natures.rb contains utility methods for page_layouts, so I've added
utility functions 'has_render_layout' and 'render_layout' there.