Fixes to Rcpp.package.skeleton

- move `call <- match.call()` earlier so the initial `list` fudging code
can work
- make sure the call gets constructed correctly when `example_code ==
TRUE` and `list` is nonempty
- remove overriding `example_code` when `attributes == TRUE` and
assume that the user doesn't want any Rcpp code when `example_code
== FALSE`; ie, they just want the `DESCRIPTION`, `NAMESPACE`, etc...

Closes #5, #61