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
Use AlreadyFinalized not ValueError
Update padding docs
Merge pull request #972 from public/padding-docs

Use AlreadyFinalized instead of ValueError
Merge pull request #962 from reaperhulk/rsa-enc

RSA encryption support
document exceptions for padding
Merge pull request #974 from reaperhulk/padding-docs

document exceptions for padding
Don't import the exceptions module directly.

The name "exceptions" is way too common for modules, so it's crappy to put that
name in the namespace, just import the classes we need.
Merge pull request #978 from alex/exception-import-fix

Don't import the exceptions module directly.
Use the short name in the docs
Merge pull request #979 from alex/short-link

Use the short name in the docs
dss_sig_value: Pure python conversion of (r, s) to byte stream
use pyasn1 for creating the dss-sig-value structure