Added support to override destination files on extraction for UnZip task

To improve the [UnZip task](https://github.com/mhoyer/albacore/wiki/UnZip-Task) I added another attribute `:force` to support overwriting destination files when unzipping a source .zip file.

My recommendation for the [UnZip wiki page](https://github.com/mhoyer/albacore/wiki/UnZip-Task):

The unzip task has a number of properties to find the files to zip and where to save the output. 
- **file** – this is the filename of the zip
- **destination** – Where you want the contents of the zip to be extracted to. In this example, it is the same directory as your build file.
- **force** – (optional, default=false) when set true it will delete each target file before unzipping from .zip file to destination.

```ruby
unzip do |unzip|
  unzip.destination = File.dirname(__FILE__)
  unzip.file = "Archive.zip"
  unzip.force
end
```