Relay should never throw

[drortirosh]

CURRENT BEHAVIOUR:
If can_relay() fails (return non-zero), the relay() method throws, causing the relay to pay for the transaction.
(NOTE: can_relay() is called directly by the relay before submitting the transaction, so the only case where the relay will attempt to create the transaction is if can_relay() succeeded when called off-chain)
In order to let the relays easily find such historical cases (where contract caused any relay to pay for unprocessed transaction) this should be fixed:

SUGGESTED BEHAVIOUR
The relay() function should be "nothrow": in case can_relay() fails, it should emit TransactionFailed( sender,target,relay, can_relay_result ) event, and exit.
Note that even if can_relay reverts, this event should be created.

With this fix, Issue #76 becomes must simpler to implement