Added a :style option for rendering partials with RecordIdentifier

Added a :style option for rendering partials with RecordIdentifier, allowing the same object to be rendered with a different partial to suit different rendering situations.

The value of :style is used to add a suffix to the standard partial name provided by RecordIdentifier#partial_path. Examples:

# Before:
<%= render :partial => "posts/post_summary", :locals => { :post => @post } %> 
# After:
<%= render :partial => @post, :style => :summary %>

# Before:
<%= render :partial => "posts/post_summary", :collection => @posts, :as => :post %> 
# After:
<%= render :partial => @posts, :style => :summary %> 

This also heavily reduces the noisy view code if you want to use an alternate partial on a collection of objects with differing classes, like records from a table with STI.