forbid RSA keys smaller than 512 bits

[Stebalien]

We do use small keys for testing but keys smaller than this are entirely useless as we need to be able to sign 256bit hashes.

fixes #42

[vyzo]

build failure:

./rsa.go:122:7: sk.Size undefined (type *rsa.PrivateKey has no field or method Size)
./rsa.go:143:7: pk.Size undefined (type *rsa.PublicKey has no field or method Size)

[Stebalien]

(need to update the go version)

[Stebalien]

Nevermind. That would require go 1.11 which I don't really want to force yet (we don't really need it). I've switched to using key.N.BitLen.