authorize → authenticate

The Foursquare api [documentation](https://developer.foursquare.com/overview/auth) states:

The examples above use /authenticate instead of /authorize, which what the OAuth protocol specifies. Following precedent established by Twitter and LinkedIn, /authenticate handles both user authentication and app authorization and automatically redirects if a user has already authorized the calling app. Conversely, /authorize will prompt the user to accept the the auth request every time.

That's why maybe, `/authenticate` is preferred.
Merge pull request #47 from hannesvdvreken/patch-1

authorize → authenticate