Merge pull request #5627 from AlbertHilb/ConditionalCallbacks

Some improvements to cell toolbars
Add some tests for the JSON validation code
Include orig_nbformat in v3 json schema
Make metadata fields be optional
Fix broken test for the invalid notebook
Update nbconvertapp.py

A spelling mistake in comment corrected in the line 109
Pygment now adds a language-class to the highlighted html so that
postprocessors can use it to identify the code block and the language
of the code inside.
Adjusted "rawcell" toolbar preset to make us of the new "register_callback"
optional parameter.
Merge pull request #5704 from dexterdev/patch-1

Update nbconvertapp.py
Merge pull request #4682 from dbarbeau/syntax-highlight

Javascript hightlighting for NBConvert html output
test notebook checkpoints in subdirectories
monkeypatch inspect.findsource in a context manager

rather than all the time, so it doesn't affect use outside IPython
Fix a NameError in IPython.parallel
Fixed latex template to support multicol w/ adjustbox.
log error message when API requests fail
log all failed ajax API requests
dialog on New Notebook failure
create checkpoints dir in notebook subdirectories
Add booktabs package to latex base.tplx
Merge pull request #5713 from takluyver/parallel-dependency-nameerror

Fix a NameError in IPython.parallel