Support for overriding destination filename

Adds support to specify the filename of the downloaded file.

Useful when downloading from servers that does redirect.

Example: `http://mvn/nexus/service/local/artifact/maven/redirect?r=snapshots&g=org.apache&a=stringutils&v=1.20&e=jar` will result in the desintation file being `redirect`.

Now you can specify `targetfilename` as `stringutils.jar`.

Example usage:

```
 pget{'Download puppet':
   source  => "http://downloads.puppetlabs.com/windows/puppet-3.4.1.msi",
   target  => "C:/software",
   targetfilename => "my-puppet-install.msi",
   username=> "myuser",
   password=> "password1'
 }
```
You modified comments but never modified to have it as a possible parameter