Proper monkey patching

The actual monkey patching method makes `abbrev` properties enumerable:

```js
require( 'abbrev' ).monkeyPatch();

var a = [ 'foo', 'bar' ], i;

for ( i in a )
    console.log( i );

/*
   Output:
    0
    1
    abbrev
*/
```

With my patch, the `abbrev` property is not enumerable anymore:

```js
/*
   Output:
    0
    1
*/
```

I also simplified an `if/else` instruction:

```js
for ( /* … */ ) {
  // …
  if ( something ) continue;
  else do_something_else;
}
```
is the same as:
```js
for ( /* … */ ) {
  // …
  if ( !something ) do_something_else;
}
```