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).
make baseProjectUrl a method in Menubar
add option in menubar to set baseproject url
make base project url a method on notebook list
change more baseurl
pass baseUrl as option
Make CodeMirror configurable

Both on a per class and per instance basis, using an option dict
in constructor and per class dict
fix default empty option
display order in output area configurable
Method to show hide linenumber of cell

mostly convenience methods to wrap codemirror one,
plus doc and 1 doc fix (missing curly bracket)
Use inputhook_wx2 on osx

Fixes #2762
doc fixes
factor valid cell index logic
refactor cellc
fix bad logic
jslint
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.