Avoid fatal error "Cannot redeclare class OAuthException"

Hi Matthew,

Thanks for a great plugin.  I hit the same "Cannot redeclare class OAuthException" error that Sven Schneider mentioned on http://matthewruddy.com/display-tweets-plugin/.  This change fixed it for me; hopefully it'll help other too (and allow me to upgrade the plugin without having to reapply the patch;) ).

Cheers, and thanks again,

Pete