Aim for whole coin change, not just cent change to avoid fees if possible

Fix #127.

v1.2 and onwards charges a fee if any output is less than 1 whole coin.  Previously the threshold was 0.01 coins.  This change changes the coin selection code to attempt to avoid creating change less than 1 whole coin.

The code is running on my live system, and made its first transaction here, making change of 1.00762073 DOGE:

http://dogechain.info/tx/bd0784287c779518abe2288883184b5cfe48d99686c76cf1adcf5c9f449d704d

The 4th input to that transaction (index 3) for 0.01164325 DOGE is an example of the change that was being created until now.