Use REST API to load notebook extensions

Use REST API as proposed in https://github.com/ipython/ipython/pull/6694 to load notebook extensions.

Example code to set configuration:
```Python
import requests
payload = """
{ 
"path" : {
    "0" : "usability/codefolding/codefolding",
    "1" : "usability/runtools/runtools",
    "2" : "usability/chrome_clipboard",
    "3" : "usability/navigation-hotkeys",
    "4" : "usability/python-markdown",
    },
"activate" : {
    "0" : true,
    "1" : true,
    "2" : true,
    "3" : true,
    "4" : true,
    }    
}
"""
r = requests.put('http://127.0.0.1:8888/api/config/nbextensions', data=payload)
```