New option "owned" in sub paste()

Hi,
I found your module very pleasant to use and added a new feature.
If you like it feel free to merge, if not, change it as you like or ignore it :-)
If there are some files (tests?) where my change would need additional changes, let me know.

This PR adds the option `owned` to the `paste()` sub, or better in `_prepare_optional_api_options()`.

When given, the `user_key` is send in the API request in the field `api_user_key`, making the paste owned by the user. Therefore, the `user_key` has to be supplied by the user and/or username/password combination must result in a positive `$api->get_user_key()` request.

**NB:** By default, at least for normal pastebin users, the resulting paste will be unlisted, even if `api_paste_private = 0`!
`api_paste_private = 2` (ie `'private' => 1` in `paste()` will still result in a private paste, though.
I did not find a way to make a listed/public paste owned by a logged in user, unfortunately. Maybe this is possible for PRO users, only?!