Add ability to append the dialog to a DOM element other than the body

This is a relatively simple change that allows the dialog to be appended to a DOM element other than the body, by specifying the desired element's id as an extra option when calling ngDialog.open.

Please be aware that:
- There are still references to the $body element that should probably be changed to the new $dialogParent element instead, especially when doing styling manipulations. It worked OK *for us*, though, so we left that untouched.
- Didn't update the minified versions.