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.
Merge pull request #872 from saschpe/master-platlib-installation

Add custom 'install' command class