add rsa_generate_private_key function to replace RSAPrivateKey.generate

refs #1026. #1101 is dependent on this
fix coverage
more coverage...
add some missing docs
rename the function
ECDSA backend
rename the docs too.
Merge pull request #1090 from public/numberless-ecdsa-backend

Numberless ECDSA backend
Simplified code
Multibackend support for EC
Add EC to changelog
Fix syntax error in docs
Merge pull request #1108 from alex/simplify-code

Simplified code
Another small cleanup
Merge pull request #1112 from public/fix-docs

Fix syntax error in docs
Documentation and interfaces for serialization to RSA*Numbers representations.
This is a function, not a method
Merge pull request #1102 from reaperhulk/rsa-generate-function

add rsa_generate_private_key function to replace RSAPrivateKey.generate
Merge pull request #1107 from public/ecdsa-multibackend

ECDSA multibackend
Merge pull request #1113 from alex/small-cleanup

Another small cleanup