Update rubocop dependency to >= 0.27
Use Twitter::Request to perform requests
Add rate_limit attribute to Twitter::Request

References https://github.com/sferik/twitter/pull/626.
Rename utility methods to be less confusing
Move Twitter::Request into Twitter::REST namespace
Move get/post from Twitter::REST::Client to Twitter::REST::Request
Create Twitter::Headers class
Rename URL_PREFIX to BASE_URL
Add leading slashes
Update default User-Agent to comply with Section 5.5.3 of RFC 7231
Replace Faraday with the HTTP gem