Allow Python-Code in Markdown Cells

Executes Python-code encapsulated in {{..}} in a markdown cell and
returns output.
Supports png, jpeg, latex and text output.
Move file
Example images
Update python-markdown

Save variables in metadata and show them after reload. Enables adding a
preprocessor for nbconvert, too.
Fix using old metadata entry

Use variables instead of cellhascode
Add markdown handling
Add text/html

Allow displaying html objects
Fix markdown handling

Marked will put text between `<p>...</p>`.
That is not desired, so remove.
Preprocessor for python code in markdown
Update custom.js example

- Use IPython.load_extensions
- Assume all extensions reside in `nbextensions`
Use contents service

Use new contents service to upload graphics, replacing the separate
websocket server.
Update runtools

- use new icon naming scheme
- update to require.js
Update navigation-hotekys extension

- fix ctrl+y
- change shift/ctrl-enter behavior to just go into edit mode after
execution
Update to requirejs
Merge pull request #87 from juhasch/example

Update custom.js example
Merge pull request #88 from juhasch/runtools

Update runtools
Merge pull request #89 from juhasch/hotkeys

Update navigation-hotekys extension
Merge pull request #86 from juhasch/contents

Use contents service
Fix render on page reload
Merge pull request #83 from juhasch/py-markdown

Extension to allow Python Code in  Markdown