Add isAccessToken to determine _accessToken set

[FrancMunoz]

Is it possible to add a function to know if access token is set?

public function isAccessToken()
{
  return $this->_accessToken ? true : false;
}

Since _accessToken is private, as it has to be, it would be great to know if it's set or not.

I did a script that does what's commented in #7 , basically a function I call to get phpParticle instance, when it returns, just calling isAccesToken() i know if getting token has gone fine or not without calling any _curlRequest function that will dramatically fail. The function takes care of token and its expiration and refreshes using refresh token.

I can make the PR if agree.

Thank you!