Add `callback` parameter to `embed_kernel` function

Can be used by caller to get connection info.
For example:
```python
def embed_cb(app):
    # sleep is needed so kernel have time to start
    os.system('sleep 3 && mate-terminal -e "ipython console --existing %s"'\
        % app.connection_file)

IPython.embed_kernel(callback=embed_cb)
```