Pound 2.6 + added PATCH (RFC5789)

I added the 2.6 version of pound and shuffled the possible values of the xhttp directive a bit:

1 now contains ``OPTIONS``, ``TRACE``, ``CONNECT``, so it has all the RFC2612 methods.
2 now contains RFC2612 and additionally the PATCH method from RFC5789

The old 2, 3 and 4 values are incremented by 1 so that they are now, respectively, 3, 4 and 5.

**This is a BC break.**