Site.copy previously didn't support multiple destinations:

   site.copy("foo", "bar1")
   site.copy("foo", "bar2")

Previously resulted in `foo` being only copied once to `bar2`.

   site.copy("static")
   site.copy("static/foo", "bar")

Previously didn't copy `static/foo` to `bar`.

This commit fixes both of these bugs. Fixes lumeland#428.