add delay to autosave in beforeunload

allows forceful 'discard changes' by reloading the page and dismissing the warning,
which @fperez really wanted today.
my attempt to fix #3735

an alternative to this would be to do something like
`\DeclareGraphicsExtensions{.pdf}` in the preamble
better message when notebook format is not supported

Different messages for bad version and bad JSON.

closes #1592
Add cwd to default templates search path.
document Ctrl-C not working in ipython kernel

This closes #2049
s/templat_paths/templat_path
Provide a more useful error for invalid use case.
The previous help statement was not very usefull for users who
don't know how to use nbconvert (ironically, the exact set of
users who would encounter the error).
Allow notebook filenames without their extensions
Added rm_math_space to markdown cells in the basichtml.tpl to be rendered ok by mathjax after the nbconvertion.
Merge pull request #3750 from jdfreder/add_cwd_templ_path

nbconvert: Add cwd to default templates search path.
Merge pull request #3760 from jdfreder/auto_nb_ext

nbconvert: Allow notebook filenames without their extensions

allows `ipython nbconvert fig` to render `fig.ipynb`
Moved print description and examples into default help output
Worked with @ivanov
fix another place we were stripping .pdf file ext
Unnecessary
Return failure, remove comments
Merge pull request #3759 from jdfreder/help_fix

nbconvert: Provide a more useful error for invalid use case.

We now print the help message if no notebooks were converted
Merge pull request #3736 from ivanov/fix-3735

Don't strip pdf extension from figure filenames

It seems to have been an erroneous assumption that this was necessary.

closes #3735
handle empty metadata in pyout messages more gracefully.
Merge pull request #3766 from StefanKarpinski/master

handle missing metadata in pyout messages more gracefully.
Merge pull request #3753 from ivanov/ctrl-c-for-kernel

document Ctrl-C not working in ipython kernel