Added sections to gallery of examples

Modified travis_script.sh to account for the new structure of the gallery

Added README.txt files in directories of gallery examples

Fixed references to gallery images in user guide pages

Fixed broken links

doc/examples/numpy_operations/plot_camera_numpy.py

@@ -0,0 +1,29 @@
+"""
+Using simple NumPy operations for manipulating images
+=====================================================
+
+This script illustrates how to use basic NumPy operations, such as slicing,
+masking and fancy indexing, in order to modify the pixel values of an image.
+"""
+
+import numpy as np
+from skimage import data
+import matplotlib.pyplot as plt
+
+camera = data.camera()
+camera[:10] = 0
+mask = camera < 87
+camera[mask] = 255
+inds_x = np.arange(len(camera))
+inds_y = (4 * inds_x) % len(camera)
+camera[inds_x, inds_y] = 0
+
+l_x, l_y = camera.shape[0], camera.shape[1]
+X, Y = np.ogrid[:l_x, :l_y]
+outer_disk_mask = (X - l_x / 2)**2 + (Y - l_y / 2)**2 > (l_x / 2)**2
+camera[outer_disk_mask] = 0
+
+plt.figure(figsize=(4, 4))
+plt.imshow(camera, cmap='gray', interpolation='nearest')
+plt.axis('off')
+plt.show()