Rewrote/reorganised SymmetricCryptoStream constructor cipher-setup logic, tidying it up.
Added cipher configuration factories to solve the protobuf-net DTO speciality class mismatch that preventing serialising objects created from the derived SymmetricCipherConfiguration objects e.g BlockCipherConfiguration.
These were convenient methods, so they were changed into static factory methods,with accompanying wrapper classes again for convenience. They may prove to be especially useful to users somewhat unfamiliar with cryptography.
This should make further packaging development and testing a lot easier.

Changed the nonce polices for several block cipher modes upon recognition of the serious security issues associated with their reuse (see: Rogaway, February 10, 2011, CRYPTREC evaluation). Further research is needed.
I may change the enumeration type NoncePolicy, and change the type to flags, so greater granularity can be achieved in recommendations and enforcement. (this is not done yet)

I also optimised the HC128 engine and acheived an extra 10 MB/s and 20 MB/s encryption and decryption speed, repectively.