move dependencies to a temporary folder on Windows (fixes #228)

When building a self-contained document, Pandoc fetches the content of HTML dependencies so that they may be inlined; that is, `<script src="foo.js">` is replaced with `<script src="data:...">`.

In pandoc 1.13 and later, Windows absolute paths beginning with a volume name are no longer supported when fetching content; that is, `<script src="\Users\bob\foo.js">` is permissible but `<script src="C:\Users\bob\foo.js">` is not.

Because it's possible for a document to reference dependencies from multiple volumes and there's no format the Pandoc dependency inliner accepts on Windows (as far as I can tell) for a path to another volume, this change works around the problem by making a temporary copy of the dependencies on Windows, so that the path can be relative. It's effectively what's already done when rendering a non self-contained document, but cleaned up with other intermediates when self-contained.