Add "page.png" and use for threshold example.

doc/examples/plot_threshold_adaptive.py

@@ -21,7 +21,7 @@ from skimage import data
 from skimage.filter import threshold_otsu, threshold_adaptive
 
 
-image = data.text()
+image = data.page()
 
 global_thresh = threshold_otsu(image)
 binary_global = image > global_thresh

skimage/data/__init__.py

@@ -96,3 +96,12 @@ def moon():
 
     """
     return load("moon.png")
+
+def page():
+    """Scanned page.
+
+    This image of printed text is useful for demonstrations requiring uneven
+    background illumination.
+
+    """
+    return load("page.png")