fix IPython.embed

without this change, IPython.embed throws the following traceback:

  File "explete.py", line 9, in dump_inputs
    IPython.embed()
  File "/home/pi/code/ipython/IPython/terminal/embed.py", line 287, in embed
    config = load_default_config()
  File "/home/pi/code/ipython/IPython/terminal/ipapp.py", line 379, in load_default_config
    for cf in Application._load_config_files("ipython_config", path=profile_dir):
  File "/home/pi/code/ipython/IPython/config/application.py", line 527, in _load_config_files
    log.debug("Loaded config file: %s", loader.full_filename)
AttributeError: 'NoneType' object has no attribute 'debug'

@Carreau was the last to poke around in this part of the code, so
pinging him, just in case
fix a NameError inside an exception
initial test for the embedding
Centering equation in markdown and code cells.
Added new structure (input prompt and inner cell) to markdown cells.
Restructuration of null.tpl and addition of inner_cell classes.
Rename of input groups and use of self.block for e_in_prompt.
Deleting html clases for raw and unknown type cells.
Simplification of the logic.
Coming back raw cell to null template.
null.tplx regenerated.
Make the name of empty_in_prompt more explicit.
Fixed mathjax macro to be synced with the mathjax config in the live notebook.
Make the null templates compatible with the last Thomas's commit.
prefer marked to pandoc for markdown2html

better consistency with live notebook
don't require mathjax on the page in mathjax utils

required for reusing mathjaxutils in node
allow utils.js to be loaded in node

where navigator is undefined
add marked.js entry point for md2html in nbconvert

now uses the exact same code as the live notebook
include marked.js in package_data
BUG: ipython_directive now supports cython magic (remove dangling space) GH4791