Set axis background to transparent in inlinebackend

Fixes #5727, related to #4608
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 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.
Make Container widgets take children as the first positional argument

This makes creating containers less cumbersome: Container([list, of, children]), rather than Container(children=[list, of, children])
Prepend a sentinel value to model ids to distinguish them from normal UUIDs (from Sylvain Corlay).
Change serialization terminology to serialize/deserialize
Update celltoolbar.js

I want to write an extension with multiple select bar. They are supposed to be displayed inline, not in block mode. The better way is to use span instead of div.
Container assumes the children attribute is not None

Thanks to @SylvainCorlay for pointing this out.
once-displayed
unnecessary context variable specify
incorrect comment
Added jquery in define
remove pagemain.js

It does not seem to be usefull anymore.

The different main.js of /tree /notebook /etc do create an instance of
`page` themselves, error pages do have hardcoded styles that show header
and sites.
Merge pull request #6064 from Carreau/fix-5727

Set axis background to transparent in inlinebackend
Merge pull request #6169 from tarzzz/fix-requirejs

Added jquery in define