Increase block prio size and max standard tx size.

This commit increases the block priority size to 50000 and the max
standard tx size to 100k.  This matches relatively recent changes in the
reference implementation.  The max block size was also increased to
750000, but since btcd does not currently create blocks, there is no
constant for it.  That constant will likely be added as a part of the
getwork implementation since it requires block creation.

Closes #71.