Add ghc-7.10.2 to travis jobs

Fix compile error for ghc-7.10.2

WIP: Plotting example

Widgets UX

Explain reasons for widget's inability to use stdin.

Add withDynamic to IPython.Kernel.ZeroMQ

Add support for stack

WIP: Rest of the widgets: Num + Box

Add link to try.jupyter.org in the README

- All widgets complete ✨
- The tutorial will need to be updated.

- Also added on_display handler (still unused though).
- Add uuidToString for UUIDs.
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- Add ChangeHandler to widgets that might want it.
- Refactor out trigger<event> functions.

- Add `properties` to view properties of widgets.
- Remove use of `Numeric.Natural`.
- Add verification mechanisms to `Attr`.