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/transform/plot_piecewise_affine.py

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+"""
+===============================
+Piecewise Affine Transformation
+===============================
+
+This example shows how to use the Piecewise Affine Transformation.
+
+"""
+import numpy as np
+import matplotlib.pyplot as plt
+from skimage.transform import PiecewiseAffineTransform, warp
+from skimage import data
+
+
+image = data.astronaut()
+rows, cols = image.shape[0], image.shape[1]
+
+src_cols = np.linspace(0, cols, 20)
+src_rows = np.linspace(0, rows, 10)
+src_rows, src_cols = np.meshgrid(src_rows, src_cols)
+src = np.dstack([src_cols.flat, src_rows.flat])[0]
+
+# add sinusoidal oscillation to row coordinates
+dst_rows = src[:, 1] - np.sin(np.linspace(0, 3 * np.pi, src.shape[0])) * 50
+dst_cols = src[:, 0]
+dst_rows *= 1.5
+dst_rows -= 1.5 * 50
+dst = np.vstack([dst_cols, dst_rows]).T
+
+
+tform = PiecewiseAffineTransform()
+tform.estimate(src, dst)
+
+out_rows = image.shape[0] - 1.5 * 50
+out_cols = cols
+out = warp(image, tform, output_shape=(out_rows, out_cols))
+
+fig, ax = plt.subplots()
+ax.imshow(out)
+ax.plot(tform.inverse(src)[:, 0], tform.inverse(src)[:, 1], '.b')
+ax.axis((0, out_cols, out_rows, 0))
+plt.show()