Allow a directory to be passed to Docker::Image#insert_local.

@windowsrefund @bfulton 

This pull allows directories to be passed to `Docker::Image#insert_local`.
For example, insert the local directory `/local` into `/image` in the container:

```ruby
image = Docker::Image.create('fromImage' => 'ubuntu', 'tag' => 'latest')
image.insert_local('localPath' => '/local', 'outputPath' => '/image')
# => returns a new image
```