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/segmentation/plot_rag_draw.py

@@ -0,0 +1,40 @@
+"""
+======================================
+Drawing Region Adjacency Graphs (RAGs)
+======================================
+
+This example constructs a Region Adjacency Graph (RAG) and draws it with
+the `rag_draw` method.
+"""
+from skimage import data, segmentation
+from skimage.future import graph
+from skimage.util.colormap import viridis
+from matplotlib import pyplot as plt, colors
+
+
+img = data.coffee()
+labels = segmentation.slic(img, compactness=30, n_segments=400)
+g = graph.rag_mean_color(img, labels)
+
+out = graph.draw_rag(labels, g, img)
+plt.figure()
+plt.title("RAG with all edges shown in green.")
+plt.imshow(out)
+
+# The color palette used was taken from
+# http://www.colorcombos.com/color-schemes/2/ColorCombo2.html
+cmap = colors.ListedColormap(['#6599FF', '#ff9900'])
+out = graph.draw_rag(labels, g, img, node_color="#ffde00", colormap=cmap,
+                     thresh=30, desaturate=True)
+plt.figure()
+plt.title("RAG with edge weights less than 30, color "
+          "mapped between blue and orange.")
+plt.imshow(out)
+
+plt.figure()
+plt.title("All edges drawn with viridis colormap")
+out = graph.draw_rag(labels, g, img, colormap=viridis,
+                     desaturate=True)
+
+plt.imshow(out)
+plt.show()