Better error checking on private key and csr.  Also, pull in file names ...

...instead of pointers.
"`type=` [in `add_argument`] can take any callable that takes a single string argument and returns the converted value" (as per [documentation](https://docs.python.org/2.7/library/argparse.html#type)) and I'd recommend to do it this way, but note my EAFP comment in the PR first