Add utility functions for checking CSR and privkey

I've added three functions to provide functionality needed to handle the TODO item at line 51 of client.py (checking validity of a CSR, checking validity of a privkey, and checking whether the two of them correspond).  This pull request does not actually implement those TODO items, just provides code in crypto_util.py that can be used to do so.

(We could also then catch IOError in client.py to indicate when one of the specified files doesn't exist or can't be read.)