Fixed Error caused by OAuthException class being redeclared.

[blizzrdof77]

Fixes Issue #19

Cause:
PHP Class "OAuthException" being redeclared if it already exists.

Solution:
Only declare if it doesn't already exist.

if (!class_exists('OAuthException')) 
  class OAuthException extends Exception {
    // pass
  }
}

Resolves the following error message:
Cannot redeclare class OAuthException