add resemble pool

src/comparison.coffee

@@ -35,7 +35,11 @@ class Comparison
   @compare: (fileAData, fileBData, callback) ->
     defer = Q.defer()
     promise = defer.promise.then callback
-    resemble.compare dataUrlHelper.toDataURL(fileAData), dataUrlHelper.toDataURL(fileBData), (data) ->
+
+    r = resemble.get()
+    r.running = true
+    r.compare dataUrlHelper.toDataURL(fileAData), dataUrlHelper.toDataURL(fileBData), (data) ->
+      r.running = false
       defer.resolve data
 
     promise

src/resemble.coffee

@@ -1,2 +1,19 @@
 Resemble = require 'phantomjs-resemble'
-module.exports = new Resemble()
+resemblePool = []
+
+module.exports =
+
+  get: ->
+    resembles = (r for r in resemblePool when r.running isnt true)
+    if resembles[0]?
+      return resembles[0]
+    else
+      r = new Resemble()
+      r.running = false
+      resemblePool.push r
+      return r
+
+  exit: ->
+    for r in resemblePool
+      r.exit()
+      r.running = false