Removed unused OTPayload::GetEnvelope, GetMessagePayload, ReadBytesFrom.

blub
Refactored OTData::Release() (does not need to be virtual).
Removed SetPayloadSize (it's the same as SetSize).
Removed GetPayloadPointer (it's the same as GetPointer).
Removed rest of unused OTPayload members.
Removed class OTPayload.
Merge pull request #282 from benma/otpayload

Remove class OTPayload