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.
Merge pull request #141 from nicksay/script-style-api

Update API to allow only one URL per name during script/style loading.
Ensure distribution files are included in the npm package.

Progress on #6 and #74.