On General Relativity

• The equivalence Principle, the cornerstone of General Relativity, says that acceleration is equivalent to force. Or as Einstein said: "we [...] assume the complete physical equivalence of a gravitational field and a corresponding acceleration of the reference system." Acceleration relative to what? Don't we need an external frame to relate this acceleration to it ??
• What about rotation? I think the equivalence of the centripetal acceleration to a gravitational field is what we call the centripetal force.
  
• How can General relativity account for the precession of a spinning wheel mounted from a rope or the precession of a gyroscope having a weight on one end of its axis??