Twitter offers a REST API which you can access using the WWW class. No additional libraries are required.
However, in order to use it you need to authenticate via OAuth and then include the OAuth headers in every request, which isn't that straight-forward.
There is a free packet on the Unity Asset Store named Let's Tweet In Unity which claims to take care ...
If you are not looking to use the SDKs you can just open a URL and the user would just have to confirm the posting. Just set a button to call the ShareToTW
private const string TWITTER_ADDRESS = "http://twitter.com/intent/tweet";
private const string TWEET_LANGUAGE = "en";
public static string descriptionParam;
private string appStoreLink = "http://www....