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

The ihaskell-widgets should send messages identical to what the
IPywidgets send. This has not been tested as there are some unresolved
issues in IHaskell that surface only if ipywidgets is installed.

Modularize build script

Removed IO, fixed JuicyPixels display