Lower min transaction relay fee to 1000 satoshi.

This mirrors a recent change in the reference implementation.  Since the
vast majority of the network still uses it and transactions which aren't
considered standard won't be relayed, the rules for standard transactions
need to be kept consistent.

Ideally this should be handled via floating fees, but we're rather limited
until we have miner support and a larger network share so the probability
of a transaction being relayed and confirmed is high.

Closes #100.

ok @owainga