Support for PSD with transparency in merged image data.

1. Using '>h' to read signed short for layer count.
2. Reading 'RGBA' for merged data with channel count 4,
and color mode 3.

For a PSD with transparency a negative layer count in
the layer info struct its absolute value is the number
of layers and the first alpha channel contains the
transparency data for the merged result.

According to:
http://www.adobe.com/devnet-apps/photoshop/fileformatashtml/PhotoshopFileFormats.htm#50577409_16000

psdparser.py

@@ -371,7 +371,7 @@ def parse_layers_and_masks(self):
             (layerlen,) = self._readf(">L")
             if layerlen:
                 # layers structure
-                (self.num_layers,) = self._readf(">H")
+                (self.num_layers,) = self._readf(">h")
                 if self.num_layers < 0:
                     self.num_layers = -self.num_layers
                     logging.debug(INDENT_OUTPUT(1, "First alpha is transparency for merged image"))
@@ -537,6 +537,8 @@ def parse_image_data(self):
             self.merged_image = self.merged_image[0]
         elif li['channels'] == 3:
             self.merged_image = Image.merge('RGB', self.merged_image)
+        elif li['channels'] == 4 and self.header['mode'] == 3:
+            self.merged_image = Image.merge('RGBA', self.merged_image)
         else:
             raise ValueError('Unsupported number of channels')