added queueLimit

Added an attribute to the Uploader called `queueLimit` with default value -1.

I'm working on a Registration page and I needed users to be able to upload one and no more than one profile picture.

Can be invoked in the controller like:

```
    uploader= $scope.uploader= $fileUploader.create {scope: $scope, url: '/upload', queueLimit: 2}

```