Add middleware to encrypt object data before sending to storage backend

[gaul]

This would enable data privacy on backends without encryption, e.g., Rackspace Cloud Files, as well as improve privacy on backends with it, e.g., Amazon S3, due to handling the private key in S3Proxy.

[shenghu]

@andrewgaul does s3proxy support encryption on object storage?

[gaul]

@shenghu S3Proxy does not support object server-side encryption due to a lack of support in the underlying jclouds, tracked by JCLOUDS-1253. This issue tracks something different, object client-side encryption. This difference lies in who does key management; in the former, the object store manages keys, in the latter, S3Proxy manages keys. Ideally S3Proxy would support both.

[gaul]

S3Proxy could include EncryptedBlobStore from Bounce:

https://github.com/bouncestorage/bouncestorage/blob/master/bounce/src/main/java/com/bouncestorage/bounce/EncryptedBlobStore.java