Added detached SecretBox

- Added SecretBox.CreateDetached() (crypto_secretbox_detached)
- Added SecretBox.OpenDetached()  (crypto_secretbox_open_detached)
- Added test
- Added new object: Detached as return type
Added detached PublicBox

- Added PublicKeyBox.CreateDetached() (crypto_box_detached)
- Added PublicKeyBox.OpenDetached() (crypto_box_open_detached)
- Added test
Merge pull request #66 from bitbeans/master

Added detached SecretBox