stats: Add http500, memoryUsage, pendingEdits gauges

and turn edits metric into a timer instead of a simple meter

src/node/server.js

@@ -23,10 +23,15 @@
 
 var log4js = require('log4js')
   , async = require('async')
+  , stats = require('./stats')
   ;
 
 log4js.replaceConsole();
 
+stats.gauge('memoryUsage', function() {
+  return process.memoryUsage().rss
+})
+
 var settings
   , db
   , plugins