Release v6.0.0

BREAKING CHANGES:

• #228 change the impersonation-related DSL
• #213 rename config.url to config.base_url
• #214 reorganize structure of the SDK, Castle::API::Session renamed to Castle::Session
• #216 add new context and payload builders, changed DSL

Enhancements:

• #231 allow to instantiate the configuration
• #230 add webhooks verification
• #223, #224, #225, #226, #227 allow to manage the devices
• #221, #222 add more tests
• #220 update the default timeout
• #218 add logger config option
• #212 drop origin from the default context