Implement gravatar to users table

Added "gravatar" column to users table. Added #gravatar_src method to user class with a before_save callback to generate a link to gravatar image for each user based on their email address. Default image size is 80x80px. To change size, append "?s=#{image_size}" to link (if you wanted a 200x200px image, append "?s=200" to gravatar link).