Created no-label-var rule.

The no-label-var rule disallows creating a label that shares an identifier with a variable.

The following patterns are considered warnings:

```js
var x = foo; 
function bar() { 
x: 
    for (;;) { 
        break x; 
    } 
}
```

The following patterns are considered okay and do not cause warnings:

```js
// The variable that has the same name as the label is not in scope.

function foo() { 
    var q = t; 
}

function bar() { 
q: 
    for(;;) { 
        break q; 
    } 
} 
```

See: http://jslinterrors.com/a-is-a-statement-label/

Fixes #29