RSA encryption support
update changelog
change doctest to codeblock until we add multibackend support for encrypt
determine operation type by isinstance on the key
improve style in test, update docs for rsa encryption review
another style fix in the rsa tests that I missed last time...
move ct length check into decrypt function, address review comments
update some decryption tests
Merge pull request #962 from reaperhulk/rsa-enc

RSA encryption support