don't use contains in SelectWidget item_query

gives false positive matches when some values are substrings of others
Check for Python errors in js tests

When running a cell in the js tests,
this checks for any errors and treats them as a failure.

Helps avoid confusion caused by silently ignoring failures in Python code.
Use 'is' to compare with None

Issues were detected by PyCharm Community Edition 3.4.
Make sure DOMWidget _css is set after basic style traits
Updating iopubwatcher.py to query for 'text' instead 'data' in the message dict.
Merge pull request #6642 from JIghtuse/master

Use 'is' to compare with None
Merge pull request #6634 from minrk/select-contains

don't use contains in SelectWidget item_query
Merge pull request #6646 from ngoldbaum/iopubfix

Updating iopubwatcher.py to query for 'text' instead 'data' in the message dict
Merge pull request #6637 from minrk/notice-python-error

Check for Python errors in js tests
Merge pull request #6645 from jdfreder/css_order

Make sure DOMWidget _css is set after basic style traits
use jsonschema refs, no need for jsonpointer
Use Draft4 JSON Schema for both v3 and v4

no longer need jsonpointer
fix some validation bugs in v3

Python API had many inconsistencies with the actual spec
raise better ValidationError in validate

when oneOf would fail on a cell or output with an unhelpful error,
validate again against the target spec,
based on the output or cell type.
add `nbformat.writes(version=X)` for downgrade

allows easy downgrade of notebooks.
support downgrading notebooks with nbconvert

Use:

    ipython nbconvert --to notebook --nbformat 3 notebook.ipynb

to write `notebook.v3.ipynb`

Adds `output_suffix` resource, inserted before file ext.
Suffix is used if `--output` is not specified.

Default is `.nbconvert`, to prevent `--to notebook` from clobbering existing notebooks by default.
add assert_big_text_equal

for better output when large strings differ
add dialogs for failed save/load

- failed validation doesn't prevent save/load,
  but the user is now warned about it.
- there is now a warning dialog if fromJSON fails,
  rather than the usual silent failure.
assume valid if there's no schema (v2)
skip additionalProperties validation on notebooks from the future

adding new keys is considered backward-compatible change,
so these additional keys should be ignored.