Simplify body & header callbacks

[bluesmoon]

curl_write_cb and curl_header_cb both assume that the data passed in as context will be a Channel and write appropriate objects to it.

We can simplify this logic by passing in a Julia function that accepts the data as its first argument and any context as a second argument. Callers could then do their own data handling without waiting for the intermediate Channel processing.

Note: This extends the request from #7