Add a base class for http async clients; refine examples.

example/http_client/CMakeLists.txt

@@ -0,0 +1,4 @@
+add_executable(http_client main.cc)
+
+target_link_libraries(http_client webcc ${Boost_LIBRARIES})
+target_link_libraries(http_client "${CMAKE_THREAD_LIBS_INIT}")

example/http_client/main.cc

@@ -0,0 +1,29 @@
+#include <iostream>
+
+#include "webcc/http_client.h"
+#include "webcc/logger.h"
+
+static void PrintError(const webcc::HttpClient& client) {
+  std::cout << webcc::DescribeError(client.error());
+  if (client.timed_out()) {
+    std::cout << " (timed out)";
+  }
+  std::cout << std::endl;
+}
+
+int main() {
+  WEBCC_LOG_INIT("", webcc::LOG_CONSOLE);
+
+  auto request = webcc::HttpRequest::Make(webcc::kHttpGet, "/get",
+                                          "httpbin.org");
+
+  webcc::HttpClient client;
+
+  if (client.Request(*request)) {
+    std::cout << client.response_content() << std::endl;
+  } else {
+    PrintError(client);
+  }
+
+  return 0;
+}