fix macOS cabal magic installation

These messages are used by console frontends to check whether the
current input is complete or not. If it is not complete, the next line
is treated as the continuation of the code, and a '...' prompt is used
to show that.

To provide support for qtconsole, I have made it such that IHaskell
always responds to an is_complete_request by saying that the code is
complete, which only allows for single line inputs.

New message types for ipython-kernel

Added some directives about using stack to manually install IHaskell,…

Added some directives about using stack to manually install IHaskell, and how to deal with the libiconv issue in OS X.

+ Some minor formatting fixes.
+ Add IHaskell.Display.Widgets.Types to hindent ignored_files.
+ Remove some extensions and unused imports.

+ Add ChangeLog.md
+ Modify `Introduction to Widgets.ipynb` to reflect changes to
`properties`

Change error message from an uncaught exception to an actual error
message.

+ hmatrix >= 0.10 && <0.17,
+ singletons >= 0.9.0 && <2.0.0.0

Found issues with the PlaceProxy implementation. (Unresolved)

* [BUGFIX]: Fix bug with width, height etc. values

The values needed to be explicitly sent as "<value>(px/em/...)" strings.
Solved by assuming "px" for all values.

* Updated messaging specification documentation as necessary.

Adding new widgets

+ Valid: A widget to display boolean values
+ Proxy: A widget that contains at most one DOMWidget in it
+ PlaceProxy: A Proxy widget with a selector (string) value.

Will have to investigate how the selector is used by PlaceProxy.

Make widget messages match with IPywidgets