change bit_strength to bit_length (bit_strength aliases still available)
modify AuthorityKeyIdentifier to support self-signed creation

Refactored the AuthorityKeyIdentifier class to take a :public_key and an
:issuer_subject rather than the previous requirement of an existing
certificate. fixes #53.

Add AKI and SKI to both self-signed and non-self-signed certificates by
default (fixes #54)