Implemented issue #998: Added support for IPv6

- Rewrote the xdebug_create_socket function so that it also supports
creating sockets for IPv6 addresses.
- Wrote a couple of tests that test both IPv4 and IPv6 connectivity.
- Added some functions to the DebugClient (some refactoring to make it
more extendable).
- Created a DebugClientIPv6 class that extends from DebugClient. This
communicates with Xdebug over IPv6.

### Known potential problems ###
- Haven't been tested on Windows.
- The DebugClientIPv6::isSupported() function isn't tested, so the SKIP for the test could be ignored on a machine that doesn't support IPv6.