Add Math display support for qtconsole

This is motivated by this SO question:

http://stackoverflow.com/q/24223695/438386

Right now is quite cumbersome to display latex in qtconsole (as my answer to that question shows), so this PR makes it work as easy as in the notebook.

Ping @asmeurer in case this helps Sympy to simplify things a bit.
Maybe printing a message about the lack of matplotlib and latex would be of better help here?