send idle/busy on all shell messages

Instead of just those that execute code.
This means that kernel_info_request triggers busy/idle,
allowing clients to check IOPub channel status without executing code.
Set axis background to transparent in inlinebackend

Fixes #5727, related to #4608
set busy/idle around every message

not just shell
Updated license to include CC BY 4.0 for documentation, examples, and other materials
Added examples to BSD, and added links to online versions of CC-BY
Moved the list of links of the CC-BY licenses below (I could not get RST to render correctly)
Check for pids when listing nbserver processes
Use check_pid from utils in IPython.parallel
update hmac signature comparison

see https://docs.python.org/2/library/hmac.html#hmac.compare_digest
Solving #5997: instances of WidgetView (that are not instances of DOMWidgetView) do not trigger widget_area.show anymore.
support Python older than 2.7.7
minor cleanup
warn users running older versions of Python
Only allow iframe embedding on same origin.
Incompatibility notice on iframe embedding
Update widget serializer/unserializer to allow custom serialization on a trait-by-trait basis.

Serializers/unserializers are given as the trait metadata keys `to_json` and `from_json`.
Give subclasses of the Instance traitlet the option of overriding the klass variable to have a default klass.

This makes it easy to construct traitlets for specific object types and default metadata.  For example, we can have a traitlet for numpy arrays that gives default values for `to_json` and `from_json`:

```python
def np_to_list(a):
    if a is not None:
        return a.tolist()
def np_from_list(a):
    if a is not None:
        return np.array(a)

class NDArray(Instance):
    klass = np.ndarray
    metadata = {'to_json': np_to_list,
                'from_json': np_from_list}
```

Then the NDArray traitlet will by default have the klass and some custom metadata.
Merge pull request #6115 from SylvainCorlay/widget-area-show

Instances of WidgetView do not trigger widget_area.show anymore.
Merge pull request #6120 from rgbkrk/no_iframe_embedding

Only allow iframe embedding on same origin by default.
remove rackcdn https workaround for mathjax cdn

cdn.mathjax.org works over https now