Add simple stats endpoint

As a beginning of #35, this implements an MVP stats endpoint that just
exposes the date that the data was processed.

cc @adborden @KyleW @mikeubell

build/stats/index.json

@@ -0,0 +1,3 @@
+{
+  "date_processed": "2016-10-10"
+}

process.rb

@@ -130,3 +130,9 @@ def build_file(filename, &block)
     ))
   end
 end
+
+build_file('/stats') do |f|
+  f.puts JSON.pretty_generate(
+    date_processed: Date.today
+  )
+end