use config instead of App.instance to propagate notebook_dir

Should behave more logically (I hope).
don't create notebook_dir if it doesn't exist
Merge pull request #5170 from jdfreder/widget-throttle-trait

Added msg_throttle sync=True widget traitlet
Merge pull request #5165 from takluyver/i5164

Don't introspect __call__ for simple callables
Moved widget files into static/widgets/*

Since they do not depend on anything ntoebook related
Updated require references to point to new files
Merge pull request #5116 from minrk/os_path

reorganize who knows what about paths
Merge pull request #5132 from minrk/oops-pdf-transform

remove application/pdf->pdf transform in javascript
Merge pull request #5136 from minrk/interact-default

set default value from signature defaults in interact
Merge pull request #5181 from minrk/gnureadline

use gnureadline on OS X
Merge pull request #5187 from minrk/ensure-metadata

handle missing output metadata in nbconvert
Merge pull request #5112 from minrk/completion-band-aid

Band-aid for completion in the notebook: not the ideal solution, but will have to do the job for now so that typing while completing works.

closes #4860
add ascii_only filter
require latex titles to be ascii

otherwise the ucs package barfs all over itself
If there is an error comparing traitlet values when setting a trait, default to go ahead and notify of the new value.
handle nodejs executable on debian

closes #5201
add @onlyif_any_cmd_exists

like @onlyif_cmds_exist, but pass if any is available instead of all
run html nbconvert tests if node or pandoc is available
inconsequential cleanup pass on test_nbconvertapp
suppress warnings test_pandoc