Make Form::open() create an HTML action attribute if the "action" array...

...item is not a controller action but a valid URL.

"Action" is the attribute name we are used to write on HTML tags, but Laravel 4 is using this same name to point to controller actions, only, I think it won't harm if people can also use it to pass an url.

Will provide valid HTML action attributes for
```
Form::open( array('action'=>'http://mysite.com/login') )
Form::open( array('action'=>URL::route('login'), 'method'=>'GET') )
```
And also for 
```
Form::open( array('action'=>'UserController@edit','method'=>'PUT') )
```