Capture standard error from curl shell command

[jaydeesimon]

If there is a nonzero exit status code from the curl shell command, there is an error message written to standard error. This pull request captures the error message, if one exists, and adds a key for it in the resulting response object.

[borkdude]

@jaydeesimon I made a couple of changes in 57ffee2.

• Rename the key to :err, same as clojure.java.shell/sh, does that make sense?
• Pass the stream if the request if asking for a stream with :as :stream. Else slurping the error output blocks.
• Added documentation