Allow dnssec key algorithm to be specified

Allow dnssec key algorithm to be specified instead of hardcoding HMAC-MD5. The
default value remains HMAC-MD5.
Remove accidental indentation
Choose default dnssec key size from key algorithm
Use dns_infrastructure_key* if present
Empty strings aren't false in Ruby
Merge pull request #243 from charlessimpson/bind-key-algorithm

Allow dnssec key algorithm to be specified