Long-lived token support and more broadcast functions

• added refreshAccessToken function that enables lib consumer to exchange refresh token for a new access token (along with username etc.)
• renamed AccessToken interface to AccessTokenResponse
• added refresh_token to AccessTokenResponse interface
• added broadcastUpvote function that broadcasts upvote for a given post to the blockchain
• added broadcastComment function that broadcasts comment to the blockchain
• added broadcastPost function that broadcasts post to the blockchain
• added broadcastPostWithBeneficiaries function that broadcasts post with beneficiaries details to the blockchain
• added createOptions helper function that creates and returns comment_options operation