Add utility function for installing mathjax for offline use

from IPython.external.mathjax import install_mathjax

install_mathjax()

* add mathjax destination to gitignore

.gitignore

@@ -4,6 +4,7 @@ _build
 docs/man/*.gz
 docs/source/api/generated
 docs/gh-pages
+IPython/frontend/html/notebook/static/mathjax
 *.py[co]
 build
 *.egg-info