DSA opaque OpenSSL
dsa docs
move where we gc dsa data, improve dsa key checking
add generate_private_key to DSAParameters + add a new function to dsa

dsa.generate_private_key(key_size, backend) will allow you to generate a
new DSA key and implicitly generate new parameters. This streamlines the
common case and will be an avenue to support future backends that don't
allow independent generation of DSAParameters (e.g. CommonCrypto)
rename backend method, add some docs
Merge pull request #1179 from reaperhulk/dsa-numbers-opaque-backend

DSA Opaque Keys for OpenSSL