Switch between Board and Wordform without hiding

Instead of using classes to hide the Board or Wordform components,
conditionally display one or the other based on this.state.playing.
Use harmony transforms

The JSX transformer ships with a bunch of useful ES6 transforms that
can be enabled by using the type “text/jsx;harmony=true”. The most
useful are enhanced object literals
(https://github.com/lukehoban/es6features#enhanced-object-literals) and
arrow functions (https://github.com/lukehoban/es6features#arrows).

Also perform a small amount of cleanup with respect to spacing, and
some un-nesting of if’s in board.jsx.
Remove key property of Board

The previous change to conditionally render Board or Wordform allows us
to remove the key property that was forcing Board to rerender. An
alternative way to fix this, when Board was always being rendered but
hidden, would be to implement componentWillReceiveProps in Board and
reset the internal state.
Declare all props

Declaring propTypes is an easy way to check for programming errors and
to see at a glance which props a component requires.
Standardize quotes

Use single quotes in all javascript strings, but double quotes for
string property values in jsx.
Rename handler functions

Rename the handler functions to be more decoupled from their parents.
Wordform, for instance, doesn’t care about the game starting; it just
needs to notify its parent when the user has entered valid words.
Remove redundant params in setState calls

The object passed to setState will be merged with the existing state,
so no need to duplicate existing state properties.
Use classSet for manipulating classes

Switch to React with addons and use classSet (common alias is `cx`)
instead of manual concatenation.
Stop passing around tile component

The logic around flipping tiles and marking them as correct was
previously contained in methods that the Board called on the Tile
component. While somewhat neat, this is not idiomatic react. Instead,
the Tile class should get most of what it needs to render in its props,
and all the logic should be contained in the Board class. In general,
passing components around, calling methods on them, and accessing
another component’s props are all anti-patterns.