Support transforming getInitialState defined via arrow functions

[jsnmoon]

Howdy!

This change adds support for when getInitialState is defined with arrow functions. I've also added the corresponding test cases.

Please let me know if I missed anything!

Supported cases:

// ArrowFunctionExpression with BlockStatement body React.createClass({ getInitialState: (): {heyoo: number} => { return { heyoo: 23, }; }, }); // ArrowFunctionExpression with ObjectExpression body React.createClass({ getInitialState: (): {heyoo: number} => ({ heyoo: 23, }), });