Integration of Sympyprt extension from IPyXt into SymPy

This PR integrates the sympyprt extension for prettyprinting sympy expressions in IPython (notebook/qtconsole) into SymPy. 

to be done
- [ ] output possible dvipng and latex errors in the notebook or qtconsole
- [ ] switch between dvipng and mathjax 'backend'
- [ ] doc