Fix for small change outputs

With the separation of CENT and MIN_TX_FEE, it is now reasonable
to create change outputs between 0.01 and 0.0005, as these are
spendable according to the policy, even though they require a fee
to be paid.

Also, when enough fee was already present, everything can go into
a change output, without further increasing the fee.
Merge pull request #428 from sipa/mintxfeefix

Fix for small change outputs
Merge commit 'a7dd11c6dadeb17cf5444dd29f3a63d80ef4b159' into namecoin_merge_v0.8
Merge commit 'b6d19aefa0cf455dca57ca74eeb75e158a9390d0' into namecoin_merge_v0.8