Updated Miso plugin

Added an option to download the image of the film. (this requires Nokogiri gem*)
Use the published date of the Miso post as the date of the entry instead of using the actual date (when Slogger was running)
Don't put the date in the content of the entry because now is in the published date.

*the image is in a namespaced tag <miso:image_url> and I can't make the default parser recognize this. 
Let me now if anyone has a solution ;)