add mapping support

mapping can be set in config.groovy to create shortcuts to url
templates.  UrlUtils was created since isUrl was used in multiple
places.

`~/.lazybones/config.groovy` might look something like

```groovy
templates {
    mappings {
        customRatpack = "http://dl.dropboxusercontent.com/u/29802534/custom-ratpack.zip"
    }
}
```