PyPy 1.4 released

When comparing the performance, we use CPython 2.6 instead of 2.5 because it is nowadays the version of CPython most commonly installed. It is also a bit faster than CPython 2.5, so benchmarking against the older CPython 2.5 would be somehow cheating. Performance is unrelated to the set of Python features available; we are working on a PyPy with Python 2.7 compatibility, but this will not have any serious impact on the performance of PyPy.