Core: small optimization (avoid looking for measure sinks if the configuration parameter is an empty string)

GCP: added class to use for implementing synchronized operations (e.g., for waiting until the starting of an instance is done)

AWS: added class to use for implementing synchronized operations (e.g., for waiting until the starting of an instance is done)

GCP: modeling zones and regions as location. Core: moved some UI-related code into a separate module

GCP: now support both 'OAuth2 Client ID' and 'Service Account' authentication strategies; also, refactored some GCP's class to reduce code duplication

Major change: all classes are now confined in the 'easycloud' namespace. Also, fixed some minor bugs.