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 that
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.
remove advice to patch by hand
Merge pull request #40 from Carreau/master

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