Add support for encrypted EBS volumes

This pull requests adds support for the newly released features for server-side encryption of EBS volumes, as described in <http://docs.aws.amazon.com/AWSEC2/latest/UserGuide/EBSEncryption.html>.

This pull request was forced to update the EC2 API version to 2014-05-01; Amazon rejected the API requests saying that there was no such field "Encrypted" if it wasn't changed. This may affect other APIs, so this should be looked at closely.

This pull request also adds unit tests for all the affected functions (get_all_snapshots, get_all_volumes, create_volume).