Template for ipynb

Template for ipynb to make lessons have the same style of others
markdown lessons and necessary changes in Makefile since now
ipynb -> md -> html.

This come from @gvwilson.

Related to issues:

- [site/119](https://github.com/swcarpentry/site/issues/119)
- [site/321](https://github.com/swcarpentry/site/issues/321)
- [bc/179](https://github.com/swcarpentry/bc/issues/179)
Some improves in template

- Add input mark
- Add output mark

BUG:

- `output.text` in `stream` block end with new line character
Fix for ipynb template

- Escape all text input and output. This is require to avoid
  troubles with `<`, `>` and other characters that have special
  meaning in HTML
- Replace markdown syntax for images output with HTML
Merge pull request #252 from r-gaia-cs/ipynb-template

Template for IPython Notebook