using() now accepts variable length of arguments, instead of 2

Changed using() so it accepts N arguments, where the last argument needs to be a callable. Arguments 1 to N-1 are all considered input. Minimum number of required arguments are 2. Throws an exception called with incorrect number of arguments, or if no callable callback was provided.

Renamed IDisposable to DisposableInterface (more common practice in PHP).

Updated tests to make use of new syntax.