Implementation of Tizen SSO APIs

Implementation of SSO APIs as per widl file https://code.google.com/p/accounts-sso/source/browse/widl/signon.widl?repo=libgsignon-glib

Backend that provides core functionality is gSSO and more information about that
component could be found at https://01.org/gsso
lots of dup code of declare and usage of `identity`. Please rework as
```
SSOIdentityPtr identity = GetIdentity(v);
if (!identity) {
  PostError(v, "Invalid Identity Object");
  return;
}

if (cmd == "startSession") {
  identity->HandleStartSession(v);
} else if (cmd == "destroySession") {
....
} else {
  std::cerr << "Received unknown message: " << cmd << "\n";
}
```