Isolate arguments into their own class and allow them to be validated

See [issue #23](https://github.com/ulrichsg/getopt-php/issues/23)

This changes the API of Option, introduces the Argument class and modifies OptionParser, CommandLineParser and their tests to use the new API.
Arguments are now also validated if they have a validation function set.

+ Argument objects can be added to an option ([22e0be5](https://github.com/jochenvdv/getopt-php/commit/22e0be5f8ee01de1e98b06663ccf004f608540a2))
+ Arguments can optionally be validated by setting a function which receives the parsed argument and returns a boolean ([fddcbfd](https://github.com/jochenvdv/getopt-php/commit/fddcbfd88538464b7c25eb2de848e6f96129823e))

If you have any questions or remarks, let me know.
I'm quite new to Git and this is my first ever real pull request, so forgive me for any oddities.
I will read up on branching and other Git techniques.