New snapshot: plugins, image thumbnails

[Dmitry Borodaenko]

I've added a configurable plugins system, for now there's only two
plugin APIs: content_inline and content_file, for handling and
rendering inline and multimedia content. See plugins/loader.rb on how
they're implemented, and models/content.rb to see how they're used.

Image plugin (plugins/image.rb) now uses ImageMagick (via RMagick
library) to generate and display reduced versions of large images in
short and full mode.

-- 
Dmitry Borodaenko