tooltip on in-place functions, julia lang hint in metadata

ship (for now) our own version of tooltip.js to have tooltip on mutating
function:

    permute!(<tab> now show a tooltip in notebook.

add a

    "language" : "Julia"

Hint in metadata to help distinguishing .ipynb file that are python from
the julia one for external program. (nbviewer/nbconvert could be modify
to use that and have correct syntax highlighting later). This would be
deprecated in favor of an official way to set the language in later
notebooks versions.

----- 

Not sure how to tell Pkg2 to install from my own fork, so I didn't try to the install process. 
Still the tooltip and metadata thing work when the files are places manually in the right directory.

I don't find the issue where we spoke of Bang and tooltip anymore, so I didn't linked it.

BTW, the scaling of the logo done by the browser is not as good as dooing it manually and copying a version with 24pixel height into the profile.