Support for sub-folders when using --preserve

Links that point to documents in sub-folders will now generate the
correct url when using the --preserve option with generate.

For example.

    gollum-site generate --preserve

Before this commit:

    [test|api/v1/test] -> href="test.html"

After this commit:

    [test|api/v1/test] -> href="/api/v1/test.html"