Add custom 'install' command class

CFFIBuild adds ext_modules late in the configuration phase. However, distutils default 'install' command also parses setup.py and thus misses that cffi contains several extension modules rather than being pure. As a consequence, add a CFFIInstall with a proper finalize_options method and use it.

In short, fixes installation to /usr/lib64/python/... rather than /usr/lib/python/....