fix(writer/spritesheet): fix variables names

fixes #1

lib/writer/spritesheet.js

 'use strict'
 
 var fs = require('fs-extra')
+  , generate = require('node-sprite')
   , lodash = require('lodash')
   , path = require('path')
-  , sprite = require('node-sprite')
   , util = require('util')
-  , utils = require('../utils')
   , Writer = require('../writer')
 
 
@@ -92,9 +91,10 @@ function SpriteSheetWriter(metadata, options, filenames) {
 
     function ondone(err) {
       if (error) {
-        return
+        return;
       }
-      else if (err) {
+
+      if (err) {
         error = err
         self.emit('internalError', err)
       }
@@ -105,7 +105,7 @@ function SpriteSheetWriter(metadata, options, filenames) {
   }
 
   function createSpriteSheet() {
-    sprite.sprite(sheetName, spriteOptions, function(err, globalSprite) {
+    generate.sprite(sheetName, spriteOptions, function(err, globalSprite) {
       if (err) {
         return self.emit('internalError', err)
       }
@@ -115,6 +115,7 @@ function SpriteSheetWriter(metadata, options, filenames) {
         , sheetExt = path.extname(sheetFilename)
         , filename
         , i = 0
+        , sprite
         , spritePath
 
       while (filename = filenames[i++]) {