Add utility to measure time

usage

```c++
jsk_topic_tools::TimeAccumulator acc;

void callback(const some::message::ConstPtr& msg) {
  jsk_topic_tools::ScopedTime time = acc.scopedTimer();
  // do something awesome
}

void printTime() {
  std::cout << "average time: " << acc.mean() << std::endl;
}

```