Remove glog dependency from stout.

[bmahler]

There are still currently numerous places in stout that use LOG functionality from glog.

We should remove these.

One example:

// Looks in the environment variables for the specified key and
// returns a string representation of it's value. If 'expected' is
// true (default) and no environment variable matching key is found,
// this function will exit the process.
inline std::string getenv(const std::string& key, bool expected = true)
{
  char* value = ::getenv(key.c_str());

  if (expected && value == NULL) {
    LOG(FATAL) << "Expecting '" << key << "' in environment variables";
  }

  if (value != NULL) {
    return std::string(value);
  }

  return std::string();
}

Becomes:

// Looks in the environment variables for the specified key and
// returns a string representation of its value.
inline Option<std::string> getenv(const std::string& key)
{
  char* value = ::getenv(key.c_str());

  if (value == NULL) {
    return None();
  }

  return std::string(value);
}