Better error messages when using Animated.Code

[hrastnik]

Added improved error messages to Animated.Code when wrong props are passed to the component. Also, unified the types both the exec and children can accept. Both exec and children can now accept a node or a function returning a node.

Example:

const node = block([...]);
const nodeFn = () => node;
// All of the following lines are now valid
<Animated.Code exec={node} />
<Animated.Code exec={nodeFn} />
<Animated.Code>{node}</Animated.Code>
<Animated.Code>{nodeFn}</Animated.Code>

I've created some tests to assert the error is being thrown when wrong prop types are provided. I've create a separate suite for testing Animated* components. I'm not really skilled in writing tests so this might be wrong.

Also, I've added the jsx-uses-react eslint rule to prevent false negatives when React is imported without using it directly, as JSX is using React implicitly. More info here: https://github.com/yannickcr/eslint-plugin-react/blob/master/docs/rules/jsx-uses-react.md

[osdnk]

Thanks!