Reorganize MessageEncryptor

1) According to OpenSSL's documentation, cipher.random_iv must be called
   after cipher.encrypt and already sets the generated IV on the cipher.

2) OpenSSL::CipherError was moved to OpenSSL::Cipher::CipherError in
   Ruby 1.8.7. Since Rails 4 requires at least Ruby 1.9.3, support for
   the old location can be dropped.