Allow a URL target for outputScript and outputStylesheet

both outputStylesheet and outputScript should be able to take a URL instead of a
resource target - there are times when you want to serve things from someone
else's machine, such as Yahoo or Google.

One use case for this change: You are writing a composite component, which uses
Google's service to load CSS (such as when you use the YUI components). This
means that you currently must either require users to put a `` tag in the
header, which is clearly undesirable, or that you need to use only local files,
which can in many cases offer worse performance.

#### Environment
Operating System: All
Platform: All
#### Affected Versions
[2.1]

Start by tracing how outputScript and outputStylesheet currently resolve resource targets. Check how a URL target could be distinguished from an existing resource target, then verify that external script and stylesheet URLs are rendered correctly for the composite-component use case described in the issue.

Written by the indexing model from the issue text.