fix: allow JS test to run after shutdown test

shutdown notebook test closes casper browser, here we re-instatiate it
after the test
Simplify StreamCapturer for subprocess testing

Rather than using a transient pipe for each subprocess started, the
StreamCapturer now makes a single pipe, and subprocesses redirect their
output to it.

So long as this works on Windows (I've done brief testing, and os.pipe()
seems to be functional), this will hopefully make this much more robust.
The recent failures in ShiningPanda on IPython.parallel have been caused
by StreamCapturer.
don't delete casper instance on shutdown test

a lunch conversation with @minrk and @takluyver lead me to just make a
popup and have it be closed, instead of having to reorder the test suite
so that this test always runs last. Now it doesn't matter what order the
test suite runs in, and gives us a pattern for testing window closing
behavior should we need it elsewhere.
use signalstatus if exit status is undefined

fixes failing test_exit_code_signal on OS X
clarify exit status / signal status comment

in _process_posix
Merge pull request #4458 from minrk/process_raw_osx

use signalstatus if exit status is undefined
Don't use select() in StreamCapturer

select() on Windows only works with sockets, not regular file
descriptors.
Correct TypeError message in svg2pdf
Merge pull request #4461 from jakobgager/fix_error_svg2pdf

Correct TypeError message in svg2pdf
Correct description for Bytes traitlet type

Closes gh-4463
Clean up MANIFEST.in
Merge pull request #4465 from takluyver/manifest-cleanup

Clean up MANIFEST.in

Remove a few exclusions which no longer exist.
Merge pull request #4464 from takluyver/bytes-traitlet-info

Correct description for Bytes traitlet type
Merge pull request #4456 from takluyver/stream-capturer-simplify

Simplify StreamCapturer for subprocess testing
Merge pull request #4451 from ivanov/shutdown-js

fix: allow JS test to run after shutdown test
allow to load config from json file
json file loading, + more doc
json takes highest precedence and is loaded last
fix doc