Add TestHelper class to deduplicate some often used methods
Fix TestHelper to support streams that do not support Stream.Length and Stream.Position
Create CryptoProvider to provide symmetric algorithms

TODO: Providing asymmetric algorithms
Add custom interfaces IEncryptionAlgorithm and IPublicKeyAlgorithm and refactor CryptoProvider
Close #9
Merge pull request #10 from secloud/cryptoprovider

Cryptoprovider