Made it clearer how coins are chosen for use when making a transaction, by switching from COIN as constant
to DUST_SOFT_LIMIT.
Modified expected values in wallet tests to take into account the numerically higher transaction costs of DOGE.
Changed constant used in calculating most applicable coin to use in a transaction, from COIN to DUST_SOFT_LIMIT
Scaled up units when testing dust-avoidance in wallet.
Shifted unit in wallet tests from CENT to COIN as a base, to avoid problems with dust avoidance code.
Reworked comment around the code for identifying transactions to spend, to separate explanation of condition, and result.
Corrected comment references to "cent", with "coin" in wallet tests, to match changes to units.
Merge pull request #451 from rnicoll/1.7-dev-tx-fee

Correct unit tests for wallet