RFC: Updating dust output definition, and default fees

URL: https://github.com/bitcoin/bitcoin/pull/2100

1) Create COIN_DUST constant, to represent the dust spam limit used.

2) Decrease COIN_DUST to 0.001 BTC

Rationale: With the increase in bitcoin value (US$13.67 as of this writing), it seems reasonable to reduce the value level of which we consider "dust spam."

3) Update TX miner and relay fee defaults to 0.001 / 0.0005 BTC respectively

Rationale: Reflects growth of dust spam in unspent transaction output dataset.

Review of impact: a) Definition of COIN_DUST reduced from 0.01 to 0.001. b) Miners who mine with this code will require a fee >= 0.001 to include TX's with outputs <= COIN_DUST c) Normal clients will require a fee >= 0.0005 to relay TX's with outputs <= COIN_DUST