fixes #742, #590: add new counts controller api

app/controllers/counts_controller.rb

@@ -0,0 +1,7 @@
+class CountsController < ApplicationController
+  respond_to :xml, :json
+
+  def posts
+    @count = Post.fast_count(params[:tags])
+  end
+end

app/views/counts/posts.json.erb

@@ -0,0 +1 @@
+{"counts": {"posts": <%= @count %>}}

app/views/counts/posts.xml.erb

@@ -0,0 +1,6 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<counts>
+  <posts>
+    <%= @count %>
+  </posts>
+</counts>