Documentation and interfaces for serialization to RSA*Numbers representations.
change names of key -> numbers interfaces
change to sub-interface idea
Fixed a few typos
Merge pull request #1122 from alex/typo-fixesg

Fixed a few typos
Merge pull request #1115 from reaperhulk/rsa-numbers-serialization

RSA Keys -> Numbers Interface
backend specific RSA*Key implementation for OpenSSL
remove skip in serialization tests until we have a backend that needs it
get numbers from loaded keys in numbers test
modify rsa.RSAPrivateKey to build old style key from numbers
remove an unused method