An scp client based on the SSH client.
Unlink the tmpfile after copying.
Emit stdout & stderr only in debug mode.
Specifying an identity file now works.

The ssh agent was failing to respect the `user` option & always
overriding it with the environment variable. No longer.
Made the `scp://user@host:/path/for/files` sugary spec work.
Added a test for it.
Documented the scp variation on ssh.
Hipster whitespace.
Pulled scp out into a prototype func; made unlink async.
Got out of `tmp`'s way on tempfile name generation.
Merge pull request #2 from npm/ceej/add-scp

Add a variation of the ssh client that uses scp