make default cell type configurable

and change the default behavior back to 2.x behavior of always 'code' (for @ellsonbg).

- adds `IPython.notebook.default_cell_type`
- default is 'code' (matching IPython 2.0)
- special heuristic values include:
  - 'selected': default to selected cell (current master behavior)
  - 'above': default to cell above
  - 'below': default to cell below
 
To change the value in custom.js:

```javascript
$([IPython.events]).on("notebook_loaded.Notebook", function () {
    IPython.notebook.default_cell_type = 'selected';
    // or 'markdown' or 'above' or 'below', etc.
});
```