Allow transfers from non-activated validators

See item 6 in #675. This makes transfers more generic.

specs/core/0_beacon-chain.md

@@ -1757,7 +1757,7 @@ For each `transfer` in `block.body.transfers`:
 * Verify that `state.validator_balances[transfer.from] >= transfer.fee`.
 * Verify that `state.validator_balances[transfer.from] == transfer.amount + transfer.fee` or `state.validator_balances[transfer.from] >= transfer.amount + transfer.fee + MIN_DEPOSIT_AMOUNT`.
 * Verify that `state.slot == transfer.slot`.
-* Verify that `get_current_epoch(state) >= state.validator_registry[transfer.from].withdrawable_epoch`.
+* Verify that `get_current_epoch(state) >= state.validator_registry[transfer.from].withdrawable_epoch` or `get_current_epoch(state) < state.validator_registry[transfer.from].activation_epoch`.
 * Verify that `state.validator_registry[transfer.from].withdrawal_credentials == BLS_WITHDRAWAL_PREFIX_BYTE + hash(transfer.pubkey)[1:]`.
 * Verify that `bls_verify(pubkey=transfer.pubkey, message_hash=signed_root(transfer, "signature"), signature=transfer.signature, domain=get_domain(state.fork, slot_to_epoch(transfer.slot), DOMAIN_TRANSFER))`.
 * Set `state.validator_balances[transfer.from] -= transfer.amount + transfer.fee`.