Rule: no-else-return

The `no-else-return` rule (I am open to suggestions on a better name) will warn when it encouters an `else` block following an `if` block which contains a return statement, as that `else` block is unnecessary and its contents could be placed outside of the `else` block.

```js
function foo() {
    if (x) {
        return y;
    } else {
        return z;
    }
}
```

This rule matches the same rule from JSLint: http://jslinterrors.com/unexpected-else-after-return/