Moved book generation code into PDF directory

Start of a reorganization of files to make book more browable and to
reduce the size of links to chapters.

pdf/nbmerge.py

@@ -0,0 +1,26 @@
+"""
+usage:
+
+python nbmerge.py A.ipynb B.ipynb C.ipynb > merged.ipynb
+"""
+
+import io
+import os
+import sys
+
+from IPython.nbformat import current
+
+def merge_notebooks(filenames):
+    merged = None
+    for fname in filenames:
+        with io.open(fname, 'r', encoding='utf-8') as f:
+            nb = current.read(f, 'json')
+        if merged is None:
+            merged = nb
+        else:
+            merged.worksheets[0].cells.extend(nb.worksheets[0].cells)
+    merged.metadata.name += "_merged"
+    print current.writes(merged, 'json')
+
+if __name__ == '__main__':
+    merge_notebooks(sys.argv[1:])