rename c33s_static_pages to c33s_static_page_content

As per @c33s's comment here https://github.com/c33s/StaticPageContentBundle/issues/15#issuecomment-50463025

I've renamed the extension configuration name to c33s_static_page_content

I also renamed the StaticPageExtension to Extension. If I  used the bundle name we
would be getting dangerously close to the 255 character limit of windows file paths/names

I don't use windows, but it's probably best to be careful.