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/features_detection/plot_orb.py

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+"""
+==========================================
+ORB feature detector and binary descriptor
+==========================================
+
+This example demonstrates the ORB feature detection and binary description
+algorithm. It uses an oriented FAST detection method and the rotated BRIEF
+descriptors.
+
+Unlike BRIEF, ORB is comparatively scale- and rotation-invariant while still
+employing the very efficient Hamming distance metric for matching. As such, it
+is preferred for real-time applications.
+
+"""
+from skimage import data
+from skimage import transform as tf
+from skimage.feature import (match_descriptors, corner_harris,
+                             corner_peaks, ORB, plot_matches)
+from skimage.color import rgb2gray
+import matplotlib.pyplot as plt
+
+
+img1 = rgb2gray(data.astronaut())
+img2 = tf.rotate(img1, 180)
+tform = tf.AffineTransform(scale=(1.3, 1.1), rotation=0.5,
+                           translation=(0, -200))
+img3 = tf.warp(img1, tform)
+
+descriptor_extractor = ORB(n_keypoints=200)
+
+descriptor_extractor.detect_and_extract(img1)
+keypoints1 = descriptor_extractor.keypoints
+descriptors1 = descriptor_extractor.descriptors
+
+descriptor_extractor.detect_and_extract(img2)
+keypoints2 = descriptor_extractor.keypoints
+descriptors2 = descriptor_extractor.descriptors
+
+descriptor_extractor.detect_and_extract(img3)
+keypoints3 = descriptor_extractor.keypoints
+descriptors3 = descriptor_extractor.descriptors
+
+matches12 = match_descriptors(descriptors1, descriptors2, cross_check=True)
+matches13 = match_descriptors(descriptors1, descriptors3, cross_check=True)
+
+fig, ax = plt.subplots(nrows=2, ncols=1)
+
+plt.gray()
+
+plot_matches(ax[0], img1, img2, keypoints1, keypoints2, matches12)
+ax[0].axis('off')
+
+plot_matches(ax[1], img1, img3, keypoints1, keypoints3, matches13)
+ax[1].axis('off')
+
+plt.show()