Loosen react/jsx-handler-names rule

This is a very stylistic rule that I don't think we should be warning about. I have a component like this:

``` JSX
<form onSubmit={this.fetchData} />
```

and I'm getting warnings in the console saying:

```
Handler function for onSubmit prop key must begin with 'handle'
```

We should be disabling this, because I think the above is perfectly fine even if not called "handleSubmit".
