Previously, the kernel would die when opening a notebook with widgets
as there is no widget stored in the kernel when it starts.

It wasn't even working. The displays were being generated due to the
sendWidgetView call from display for Button.

All functions now accept a Button as their first argument.

Evaluate accumulated widget messages in the Ghc Monad, after a comm_msg
recieved from the frontend is handled.

- Move dupHeader to IHaskell.Types
- Move handleMessage and widgetHandler to IHaskell.Eval.Widgets
- Make handleMessage and widgetHandler more flexible

Buttons were seen in the notebook. It seems that an empty label was the
culprit, even though IPython somehow manages to accept empty strings.

Still no visible outputs.

Fix build issue with ghc <7.10

Remove dependencies

Get rid of system-filepath

No more classy prelude

Fix formatting in IHaskell/Eval/Evaluate.hs