Initialize OS block storage REST API clients

Package name: kamaki.clients.blockstorage
Add unit tests for Block Storage REST client
Implement Block Storage client
Add unit tests for Block Storage client
Implement Cyclades Block Storage rest client

Includes unit tests
Implement Cyclades Block Storage client

The Cyclades client overrides some methods of the OS-specific
client, due to API differences: create_volume, create_snapshot

Unit tests for these methods are also implemented
Expose volume & snapshot commands (Block Storage)