Add gh-pages update command

Author: mobiusklein

makefile

@@ -5,4 +5,14 @@ test:
 	py.test -v  brainpy --cov=brainpy --cov-report=html --cov-report term
 
 retest:
-	py.test -v brainpy --lf
+	py.test -v brainpy --lf
+
+update-docs:
+	git checkout gh-pages
+	git pull origin master
+	cd docs && make clean
+	cd docs && make html
+	git add docs/_build/html -f
+	git commit -m "update docs"
+	git push origin gh-pages
+	git checkout master