Support Implicit Grant Authorization Flow (Fix #499) (#515)

* Add SpotifyImplicitGrant with get_access_token and get_cached_token (and minimum related functions)

* Add some overlooked necessary methods/values in SpotifyImplicitGrant

* Remove unsuppported functionality and make SpotifyImplicitGrant public

* Allow/Expose integration of SpotifyImplicitGrant in client

* Add Implicit Grant tests and decrease abilities of prompt_for_user_token

Remove Implicit Grant and state support from prompt_for_user_token

* Add documentation and changelog entry

* Touch up PEP8 compliance

* Ignore long line with link for flake8

* Correct changelog

* Restore compatibility with Python 2.7

* Correct help(SpotifyImplicitGrant.get_access_token)

* Remove as_dict from SpotifyImplicitGrant.get_access_token

* Combine status check functionality with implicit grant support

In oauth2.py:
* Add state checking to SpotifyImplicitGrant
* Add dedicated SpotifyStateError as subclass of SpotifyOauthError
* Moved `_get_user_input` from SpotifyOAuth to superclass SpotifyAuthBase
* Renamed `parse_oauth_response_url` to `parse_auth_response_url`
* Moved error handling into `parse_auth_response_url`

Made minor changes in tests and client.py accordingly

* Update changelog

* Trim down tests for SpotifyImplicitGrant

* Fix trailing whitespace

CHANGELOG.md

@@ -9,8 +9,14 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ### Added
 
+ - Added `SpotifyImplicitGrant` as an auth manager option. It provides
+ user authentication without a client secret but sacrifices the ability
+   to refresh the token without user input. (However, read the class
+   docstring for security advisory.)
+ - Added built-in verification of the `state` query parameter
  - Added two new attributes: error and error_description to `SpotifyOauthError` exception class to show
    authorization/authentication web api errors details.
+ - Added `SpotifyStateError` subclass of `SpotifyOauthError`
  - Allow extending `SpotifyClientCredentials` and `SpotifyOAuth`
  - Added the market paramter to `album_tracks`