Automatically generate Samples wiki page from README files.

Reviewed in http://codereview.appspot.com/5900069/.

Index: samples-index.py
===================================================================
new file mode 100644
diff --git a/Makefile b/Makefile
index 8d6962b..3786b7f 100644
--- a/Makefile
+++ b/Makefile
@@ -19,6 +19,11 @@
 docs:
 	cd docs; ./build.sh
 	python describe.py
+	python samples-index.py ../google-api-python-client.wiki/SampleApps.wiki
+
+.PHONY: wiki
+wiki:
+	python samples-index.py > ../google-api-python-client.wiki/SampleApps.wiki
 
 .PHONY: prerelease
 prerelease: