Downvote, user metadata and revoking access token

• added a createVote function which creates and returns a vote operation
• added a broadcastDownvote function
• refactored broadcastUpvote to use createVote
• added revokeAccessToken function
• added setUserMetadata function

HOTFIX

• fixed exports

State, `createFirebaseAccount`, and broadcasts

• enabled to add state variable in the getAuthorizationUrl function as an optional parameter
• added createFirebaseAccount function
• added broadcastFollow function
• added broadcastUnfollow function
• added broadcastReblog function

New documentation

• added new documentation in the README
• added JSDocs documentation in the code

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