Refactoring

Hello! This patch introduces these changes to the script:

1. No CURL. It is faster and more robust to use `file_get_contents()`
2. Instagram no longer provides you with an access token, so the link to a access token generator is given.
3. `<?` to `<?php` since the first variant is deprecated and turned off by default in the latest PHP versions.
4. Less code (from 30 lines to 12).
5. Little optimisations (`foreach` swiched to `[0]`, only one "Location: " string, one `$return->data` call instead of two).

`README` is updated with all new information.