Read binary files as ByteString

Refactoring Widgets + Boolean Widgets

- Formatting issues: Bad numbering.
- Clarify usage of custom messages.

- Make verify_formatting.py skip Types.hs and Common.hs
- Fix formatting issues in rest of the files

- Selection Widgets
  - Dropdown
  - ToggleButtons
  - RadioButtons
  - Select
  - SelectMultiple
- Small fix to MsgSpec.md

- Rename IHaskell.Display.Widget.Types.Widget to IPythonWidget
- Add explanatory comments to the source files
- Add documentation for messaging protocol in MsgSpec.md
- Add README.md
- Remove unnecessary extensions and imports

Use default value from ghc-7.10, i.e 100.

Make the example in ipython-kernel compile again

Function objectInfo was renamed to inpectInfo

- Couldn't format as hindent errors out.
- Remove setField' from export list.
- Add version constraints in ihaskell-widget.cabal

- Widgets work, although it requires some tweaking to their properties.
- Dropping dropdown for now. Will deal with selection widgets later.
- Default properties need to be improved.

- Remove IHaskell.Display.Widgets.Common
- Partially refactor Button implementation
- Stuck with ToJSON instance for Widget

Dropdown and Output Widgets

- Add explicit export list
- Rename to DropdownWidget
- Add selection handling

- More features may be added

- Add dropdown widget and some functions to manipulate it
- Requires the ability to update the internal stored widget without
  sending a message to the frontend.

- Add support for `clear_output`
- Rename `setOutput` to `appendOutput`

- Add Output Widget to ihaskell-widgets
- Add support for sending `display_data` messages as custom messages.

Backbone widgets

- Move update and modify to Common
- Move str :: String -> String to Common
- Rename ImageInt to PosInt
- Move PosInt to Common
- Correct some comments erroneously referring to Buttons

Add ImageWidget

Use readFile from ByteString to read binary files

- Send [method .= custom] messages via widgetSendCustom
- Send arbitrary JSON messages via widgetSendValue
- Refactor IHaskell.Eval.Widgets, tidy up handleMessage

- Reformat according to hindent
- Remove String.hs

Reading a file that might contain PNG data using `System.IO.readFile`
causes `hGetContents: invalid argument (invalid byte sequence)` due to
file contents not being valid text under the current system locale.
This fixes loading diagrams into Jupyter notebooks.

- All four string widgets complete.
- Switch to using 'Text' everywhere. How'd String crop up?