Investigate what can be done for pynacl dependency on libffi (header file ffi.h)

Carrying this over from https://github.com/bigchaindb/bigchaindb-driver/issues/140, copying here for convenience:

Some questions:

* Can we detect the missing header file via `setup.py`?
* Can we package the header file as part of the distribution? See following:
* https://github.com/pypa/packaging-problems/issues/84
* http://stackoverflow.com/questions/32313793/how-to-have-pypi-package-install-header-files-for-c-extension-with-distutils-set

**Important note**: Although this issue is being created under bigchaindb-driver for the time being, this is something, if possible, that should be pushed down to cryptoconditions, at the very least, and further to pynacl if it makes sense.