Make PyCrypto work with PyPy (bug #1131452)

With this set of patches, PyCrypto works under PyPy.

**NOTE:** It would be cleaner and faster to have this done via `cffi` or via a Cython extension. Still, this is a start and at least it allows PyCrypto-based application to run under `pypy`.

The main change concerns the 2 routines `longObjToMPZ` and `mpzToLongObj` in `_fastmath.c`.
They have been completely changed so that they don't access the internals of a PyLong anymore (which are not exposed by PyPy).
Even though the new routines are less efficient, I didn't notice any significant performance degradation with CPython; that probably means these routines are not hotspots anyway. I left one of the two routines only for Python < 2.7; the alternative was to backport `PyLong_AsLongAndOverflow`.

The interesting thing is that PyCrypto Public Key functions are **slower** under PyPy compared to CPython by a 30% factor.

This patch solves this bug  https://bugs.launchpad.net/pycrypto/+bug/1131452 .

Note: In order to build the C extensions, the following patch needs to be applied to PyPy: http://web.archiveorange.com/archive/v/ly9iT75Cutf1uTbpBled . Most recent versions of PyPy include it already.