Improve IPython support

The lancet.ipython module no longer provides replacement versions of
classes for IPython support, instead it now acts as an extension which
can be loaded with `%load_ext lancet.ipython` and registers
display-formatters for classes (including child classes) which should
provide pretty-printing support and HTML support for IPyNB.
Merge branch 'master' into python3

Conflicts:
	lancet/core.py