Adds p.break_ for explicit newlines in lib.pretty

p.break_() explicitly includes a newline while pretty printing, and maintains indentation.
A simple test is included to show this working.
Uses p.break_() when repr() is used for printing

When a newline is found in the result of repr() (using the default printer), the newline is replaced with an appropriate call to p.break_()
"use strict" in most (if not all) our javascript

+ fix one variable leaking in global nameespace
Use default OS shell to run system commands

Instead of using os.system which uses /bin/sh, this uses subprocess.call (the replacement of os.system) to run the command using the default shell of the OS. With this, one can use more advanced commands for bash, zsh, ksh, ...

I do not have a win32 system to test this modification, so maybe line 2236 can also be changed like 2239.
rename call methods to transform and postprocess

- The `call` methods for nbconvert transformers has been renamed to
  `transform`.
- The `call` methods of nbconvert post-processsors have been renamed to
  `postprocess`.

  pinging @Carreau and @jdrefer, who probably knows who'd be affected by
  this change...  maybe @damianavila, @jakevdp
replace 'transformer' with 'preprocessor'
fixup minor typos from search/replace
renamed transformer to preprocessors
minor typo
capture rich output as well as stdout/err in capture_output

closes #3742
add outputs property on CapturedIO
add --no-display flag to %%capture

document this new magic in whatsnew, and added docs to the resulting object.
updated Basic Output notebook with  new %%capture
default values for RichOutput attributes
tests for utils.capture
subclass RichOutput in ExecuteReply

they are very similar

add another test for good measure.
respect `pylab_import_all` when `--pylab` specified at the command-line

Earlier cleanup cleaned up a bit too much, ignoring this configurable.

should be backported to 1.x
only strip continuation prompts if regular prompts seen first

should be discussed before merge (ping @takluyver),
as it is a significant and sensitive change.

I'm also not sure that it's the right fix, but worth a try.

closes #4059
Merge pull request #3969 from Carreau/usestrict

"use strict" in most (if not all) our javascript

plus fix one variable leaking in global namespace
store default codemirror mode in only 1 place