"norun" cell metadata

It occurred to me that it would be useful to have a `norun` code-cell metadata flag, that used to:
- Prevent the cell from being executed in the notebook interface when _Run All_ is chosen — you have to run the cell explicitly with shift-enter etc.
- Prevent the cell from being executed when it the notebook is included in another notebook via the `%run` magic in IPython.
- Prevent the cell from being executed when the notebook is included in other contexts, e.g. in my [NBInclude](https://github.com/stevengj/NBInclude.jl) package for including IJulia notebooks in Julia programs.

I have been thinking about [literate programming](https://en.wikipedia.org/wiki/Literate_programming) styles based on the notebook format, which is why I made NBInclude so that notebooks can be first-class collections of Julia code (as an alternative/supplement to `.jl` files). If I'm developing code in a notebook, I might want runnable code examples and tests mixed with the "production" code, but I wouldn't want those snippets to be executed along with the rest of the file when I include the file elsewhere.

The UI in the notebook could be a checkable "Skip Cell in Run All" menu item in the `Cell` menu, and perhaps the `norun` cells could be displayed slightly differently, e.g. with a darker-gray background.