#626: ImportError when IPython not installed

When importing pymc, "interactive_sampling.py" would try to import
IPython. If IPython is not present, importing pymc failed with an import
error.

The solution is to check for the IPython module to be present. This
commit also checks if it is running inside an IPython Notebook kernel
and raises an NotImplementedError otherwise.
Tests for 3.3 are failing because of a syntax error on this line. s/, e/as e/ should fix it.