MINOR: httpclient/lua: httpclient:get() API in lua

MINOR: httpclient/lua: httpclient:get() API in lua

This commit provides an hlua_httpclient object which is a bridge between
the httpclient and the lua API.

The HTTPClient is callable in lua this way:

    local httpclient = core.httpclient()
    local response = httpclient:get("http://127.0.0.1:9000/?s=9999")
    core.Debug("Status: ".. res.status .. ", Reason : " .. res.reason .. ", Len:" .. string.len(res.body) .. "\n")

The resulting response object will provide a "status" field which
contains the status code, a "reason" string which contains the reason
string, and a "body" field which contains the response body.

The implementation uses the httpclient callback to wake up the lua task
which yield each time it pushes some data. The httpclient works in the
same thread as the lua task.