Component type support for 'icon' prop and bugfix and update Readme

[ngxiaoyi]

1. Support component/react element type of 'icon' prop. Component type of icon prop type support #20 Now we can use react-native-vector-icons or other image component like this:

<ItemCell
  icon={<Icon name="ios-person" size={30} color="#4F8EF7" />}
>
  text here
</ItemCell>

2. Remove children proptype check. "this.props.children" is a special prop of React： https://facebook.github.io/react/docs/jsx-in-depth.html#children-in-jsx , the children prop here won't work and will leads to an warning.

3. update the doc

Tested on my local env, maybe need furthermore tests.

[alvaromb]

Hey! Looks great @ngxiaoyi!

Thanks.

[alvaromb]

Looks good, will merge today.

Thanks.