add custom.js file for notebook customisation

Highlighting should now be correct in live notebook app. At least on IPython > 1.1.0 and 2.0.0-dev.
Will **not** work but fails gracefully on 1.0.0. (syntax highlight will still by python)

Also this adds a small hint into notebook metadata that notebook is haskell that **might** be use later.
(IRuby and IJulia do the same)

Highlight will not be correct on nbviewer, but there is nothing you can do for now,
at some point I might use above hint to highlight correctly on nbviewer.