WIP Sympyprt extension from IPyXt copied to sympy

This PR copies the sympyprt extension for prettyprinting sympy expressions in IPython (notebook/qtconsole) to sympy. 

The test for the printing extension was copied as well and updated to use the sympy testing infrastructure. Note that the test does not check the generated latex code nor the png files. 

There are two functions in the unit test:
```test_sympyprt``` and ```test_sympyprt_ipython```
the first one is almost a 1:1 copy of the code in the unit test shipped in the IPyXt repo except that the ipython commands (%load_ext) were commented out. The latter function uses an ipython session. I think that ```test_sympyprt``` can simply be removed as we can't test an ipython extension (%load_ext is invalid) with it - do you agree ?

to be done
- [ ] python3 support
- [ ] check generated latex code and/or generated pngs
- [ ] doc