added /api/info/:id route

[dannycoates]

This removes the dtotal and dlimit from /api/metadata since the downloader doesn't need (and probably shouldn't have) them. It also allows us to remove the authKey from the sender's local storage.

fixes #738

[ehuggett]

since the downloader doesn't need (and probably shouldn't have)

does this mean there will be no way to warn a user (downloader) that this is the last download before the file is deleted? a boolean for 2 or more remaining downloads would probably be sufficient for the downloader

[dannycoates]

does this mean there will be no way to warn a user (downloader) that this is the last download before the file is deleted?

As is, this would do that. A boolean finalDownload or something seems ok though.

[dannycoates]

I added finalDownload to the /api/metadata response. It isn't transactional of course...