Moved the IPython extension into a subpackage for refactoring
Fixed Pyflakes warnings regarding unused imports in tests directory
Added pyflakes ignore for global import from builder.shared_conf
Refactored all IPython magics into separate ipython/magics.py file
Refactored all IPython display hooks into ipython/display_hooks.py
Made imports in IPython extension Python3 compatible
Updated docbuilder and nbpublisher submodule references