Added etag support for CSS served from the cache via scss_server

To do this I modified scss_server such that it stores imports, etag, etc. in a .meta file rather than .imports. I think .meta better describes what the file now contains. It also ensures that a new file is generated, thus avoiding the issue of backwards compatibility with .imports files.

I can't think of any issues this should cause to existing users, but I thought it worth me mentioning.