siacoin outputs in the wallet are now iterated through in sorted order,
starting with the largest output and going to the smallest output. This
stifles a DoS attack where a bunch of tiny outputs are sent to a
person's wallet, causing all of their transactions to be gigantic as the
wallet nearly always selects a large volume of small otuputs when
creating a transaction.

This commit could be further improved upon by, instead of always
selecting the largest output, selecting the smallest output that still
covers the entire balance. Such a change would reduce the average
transaction size.

fixes #526

most users should no longer have problems with this, though the problem
is not completely solved.