Removed unused variable

The current version of code uses const AUTHORIZE_URL
Method get_auth_url() return encoded callback

Before: https://api.instagram.com/oauth/authorize?client_id=abc&redirect_uri=http://localhost/instagram/callback&response_type=code
After:  https://api.instagram.com/oauth/authorize?client_id=abc&redirect_uri=http%3A%2F%2Flocalhost%2Finstagram%2Fcallback&response_type=code
Merge pull request #5 from bessarabov/encoded_auth_url

Encoded auth url