introduce LWS_CALLBACK_HTTP_WRITEABLE

Signed-off-by: Andy Green <andy.green@linaro.org>
create user alloc for http if requested

Signed-off-by: Andy Green <andy.green@linaro.org>
add by hand http send example

This adds a demonstration of how to use the http nonblocking
send action to the test server.

If you ask for /leaf.jpg from the test server, it will send
"by hand" a 2.4MB jpeg in HTTP, including the headers.  See
the test server sources to see how it is done.

Although it's a file, and a jpeg image, actually the exact
same scheme will work for any data or mime type.

Signed-off-by: Andy Green <andy.green@linaro.org>