use m2crypto instead of rsa

The pubkey is converted from PKCS#1 to X.501 format to be usable by
M2Crypto.

To test the change add an entry in your config.yml's dev flavor
which looks like this:

privileged_key: /path/to/pubkey

	modified:   lib/config.py
	modified:   registry/toolkit.py
	modified:   .travis.yml
	modified:   README.md
	modified:   requirements.txt