Cleanup automation client

need to cleanup and restructure automation client code.

• put tests in nuxeo-automation-test
• put common code in nuxeo-automation-client-common (used by both client implementations and server)
  (optionally rename automation-client to automation-client-http)
• split packages to be able to implement a GWT client
• improve interfaces and "spi" to ease writing new implementations
• package a hessian client/server implementation and put it in sandbox or addons?