Add tifffile to a new external package.

skimage/external/tifffile/__init__.py

@@ -0,0 +1,4 @@
+try:
+    from tifffile import *
+except ImportError:
+    from ._tifffile import *

skimage/io/_plugins/pil_plugin.py

@@ -14,7 +14,7 @@ except ImportError:
 from skimage.util import img_as_ubyte, img_as_uint
 
 from six import string_types
-from tifffile import imread as tif_imread, imsave as tif_imsave
+from skimage.external.tifffile import imread as tif_imread, imsave as tif_imsave
 
 
 def imread(fname, dtype=None):

skimage/io/_plugins/tifffile_plugin.py

@@ -1,6 +1 @@
-try:
-    from tifffile import imread, imsave
-except ImportError:
-    raise ImportError("The tifffile module could not be found.\n"
-        "It can be obtained at "
-        "<http://www.lfd.uci.edu/~gohlke/code/tifffile.py>\n")
+from skimage.external.tifffile import imread, imsave