Decrease COIN_DUST to 0.001 BTC, Update TX miner and relay fee defaults

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.