Label for custom css styling of checkbox

[at0mat]

Using css to override styling of a checkbox requires a <label></label> as a sibling (same level, next element) to the <input type="checkbox" /> node.

Without using a label with the htmlFor attr, mouse clicks don't propagate to the checkbox input, and the handleChange() callback never fires.

I was unable to make the click events propagate using the existing <span className={${prefixCls}-inner} />

Reference for styling approach

[coveralls]

Coverage remained the same at 93.548% when pulling 7151187e889fe666c7491c6c5a40414dc072ce9d on StocksyUnited:master into b3b17afe247060fe8435db1b868dd0d183b9d098 on react-component:master.