Add SHA256/512 module support.
Add basic support for C API attr members
GC: Attributes can have values that did not get allocated by the GC
Using the C API it's possible to create a Box instance without using our GC alloc.
For example have a look at md5module.c: PyDict_SetItemString(d, "MD5Type", (PyObject *)&MD5type);
Merge pull request #129 from undingen/sha_modules2

Add the sha256 and sha512 modules
Support for __get__ and __set__ for descriptors