Update API to allow only one URL per name during script/style loading.

To maintain backwards compatibility across versions, this change only updates
the API documentation and throws a non-blocking error if an unsupported array
of URLs is passed instead of a single URL.  Afterwards, a following change can
safely transition the implementation without affecting the API.

Progress on #25.
Singular style in unload as well. Although for all of these stylesheet would probably make more sense.