Lower the memory consumption

A pako object contains a 64k buffer. We create a `FlateWorker` for each zip
entry, meaning a zip file with a lot of entries would take **a lot** of memory.

Lazy-loading the pako object isn't the best solution but it's the quickest.
The best solution is to lazy-load the worker list.

Mitigate the issue Stuk#446.

lib/flate.js

@@ -18,21 +18,12 @@ exports.magic = "\x08\x00";
 function FlateWorker(action, options) {
     GenericWorker.call(this, "FlateWorker/" + action);
 
-    this._pako = new pako[action]({
-        raw:true,
-        level : options.level || -1 // default compression
-    });
+    this._pako = null;
+    this._pakoAction = action;
+    this._pakoOptions = options;
     // the `meta` object from the last chunk received
     // this allow this worker to pass around metadata
     this.meta = {};
-
-    var self = this;
-    this._pako.onData = function(data) {
-        self.push({
-            data : data,
-            meta : self.meta
-        });
-    };
 }
 
 utils.inherits(FlateWorker, GenericWorker);
@@ -42,6 +33,9 @@ utils.inherits(FlateWorker, GenericWorker);
  */
 FlateWorker.prototype.processChunk = function (chunk) {
     this.meta = chunk.meta;
+    if (this._pako === null) {
+        this._createPako();
+    }
     this._pako.push(utils.transformTo(ARRAY_TYPE, chunk.data), false);
 };
 
@@ -50,6 +44,9 @@ FlateWorker.prototype.processChunk = function (chunk) {
  */
 FlateWorker.prototype.flush = function () {
     GenericWorker.prototype.flush.call(this);
+    if (this._pako === null) {
+        this._createPako();
+    }
     this._pako.push([], true);
 };
 /**
@@ -60,6 +57,26 @@ FlateWorker.prototype.cleanUp = function () {
     this._pako = null;
 };
 
+/**
+ * Create the _pako object.
+ * TODO: lazy-loading this object isn't the best solution but it's the
+ * quickest. The best solution is to lazy-load the worker list. See also the
+ * issue #446.
+ */
+FlateWorker.prototype._createPako = function () {
+    this._pako = new pako[this._pakoAction]({
+        raw: true,
+        level: this._pakoOptions.level || -1 // default compression
+    });
+    var self = this;
+    this._pako.onData = function(data) {
+        self.push({
+            data : data,
+            meta : self.meta
+        });
+    };
+};
+
 exports.compressWorker = function (compressionOptions) {
     return new FlateWorker("Deflate", compressionOptions);
 };