Update snapchat_agent.php

separate isMedia into isImage and isVideo functions which can be used independently to identify if the media is an image or a video. The isMedia function relies on the individual functions and allows a consumer of this class to use these headers which are much more accurate than the media_type attribute which I have (anecdotally) found to be unreliable.

You seem to prefer the more verbose:
```php
if (test) {
  return TRUE;
}
return FALSE;
```

pattern, I didn't look very hard for contribution guidelines so I went with the a condensed style. Let me know if you have a strong preference for the other style.