Add examples.

Disable google tests for now.

examples/usbstream/usbstream.cpp

@@ -0,0 +1,11 @@
+#include "cameraserver.h"
+
+#include <thread>
+
+int main() {
+  cs::USBCamera camera{"usbcam", 1};
+  camera.SetVideoMode(cs::VideoMode::kMJPEG, 320, 240, 30);
+  cs::HTTPSink httpServer{"httpserver", 8080};
+  httpServer.SetSource(camera);
+  for (;;) std::this_thread::sleep_for(std::chrono::seconds(1));
+}