truncate potentially long CompositeErrors

If you have a typo in a task on 100 engines, the local exception will be a bit ridiculous.

This defaults the truncation to 4 tracebacks (could be even fewer, or even one?),
and you can set the tb_limit attribute of CompositeError to adjust this.

Prompted by [this SO question](http://stackoverflow.com/questions/14635935/silence-or-condence-ipython-parallel-exceptions).
Use inputhook_wx2 on osx

Fixes #2762
factor valid cell index logic
refactor cellc
Updating notebook compat docs.
abstract, cleanup and document

Separate the methods that actually insert dom element for easier
testing.

Cleanup, and order methods more logically

add "docstring"
Removing old dev docs.  All of this has been moved to GH wiki.
Adding link from Sphinx dev docs to GH wiki Dev: Index.
simplify logic
remove unused/not working function
move CM IPython theme out of codemirror folder
Don't enable pylab mode, when matplotlib is not importable

On a headless server, matplotlib is a unnecessary dependency and the ipython
console will crash badly, when tring to enable %pylab there.
In that case, just don't enable %pylab and ask the user to install matplotlib.

Original bug report: https://bugzilla.redhat.com/show_bug.cgi?id=872176
Refactor qt import logic. Fixes #2955
Add command to trim the history database.
document CompositeError.tb_limit
Merge pull request #2871 from minrk/shortexc

truncate potentially long CompositeErrors
Merge pull request #2901 from lessandro/osx_inputhook_wx

Fix inputhook_wx on osx
Merge pull request #2681 from tomspur/remove_matplotlib_dep

Don't enable pylab mode, when matplotlib is not importable
time magic: shorten unnecessary output on windows

On windows the detailed info (i.e. cpu time) is not available and
always reported as zero. The rest of the information is therefore
redundant with the next print (wall time) and can be omitted.
fix count when testing composite error output