Support Twitter API v2

[Harmon758]

This pull request adds support for Twitter API v2, resolving parts of #1472, with a Client class and its methods:

• Client.hide_reply (PUT /2/tweets/:id/hidden)
• Client.unhide_reply (PUT /2/tweets/:id/hidden)
• Client.search_all_tweets (GET /2/tweets/search/all)
• Client.search_recent_tweets (GET /2/tweets/search/recent)
• Client.get_users_mentions (GET /2/users/:id/mentions)
• Client.get_users_tweets (GET /2/users/:id/tweets)
• Client.get_tweet (GET /2/tweets)
• Client.get_tweets (GET /2/tweets/:id)
• Client.unfollow (DELETE /2/users/:source_user_id/following/:target_user_id)
• Client.get_users_followers (GET /2/users/:id/followers)
• Client.get_users_following (GET /2/users/:id/following)
• Client.follow (POST /2/users/:id/following)
• Client.get_user (GET /2/users/:id, GET /2/users/by/username/:username)
• Client.get_users (GET /2/users, GET /2/users/by)

New API v2 models have been added:

• Media
• Place
• Poll
• ReferencedTweet
• Tweet
• User

replacing API v1.1 models (which can still be accessed through tweepy.models) in the package namespace.

A Paginator has also been added for use with Client methods.

This PR also includes documentation and tests for Client, as well as documentation for Paginator and the new models.

Currently, this is in a very preliminary rough draft state, and many parts of this initial implementation might change before this pull request is complete. I wanted to get this branch and draft pull request out as soon as possible, as there have been requests to do so (e.g. by @sparack and others) and so that anyone can test Twitter API v2 and provide feedback on Tweepy's interface for it.

Note, there is still planned functionality missing in this initial implementation, such as error and rate limit handling, a pagination interface, and methods for the newest manage follows endpoints.
Documentation and tests also have not been added yet.
I'm also still considering how to expose tweepy.user.User, so as not to conflict with the API v1.1 User object/model.

[Harmon758]

This branch/pull request should be complete now.
I plan on adding methods for the new Follows and Blocks endpoints (see #1472) after this has been merged.
Streaming will probably be a separate PR.

Any feedback and/or review from @sparack or anyone else would be appreciated.