Interactive ipython printing.

I am collecting here some ipython printing issues and trying to resolve them.

Issue 2945: Make init_printing() work with latex printing in the IPython qtconsole and notebook
http://code.google.com/p/sympy/issues/detail?id=2945

Issue 3537: %load_ext sympy.interactive.ipythonprinting int and float representation
http://code.google.com/p/sympy/issues/detail?id=3537

Issue 3728: garbage from preview() in ipython console session
http://code.google.com/p/sympy/issues/detail?id=3728

Issue 3777: IPython pretty printing should default to MathJax
http://code.google.com/p/sympy/issues/detail?id=3777