Fix bug #152: NpgsqlInterval different constructors different ToString

Take the days from the Timespan, then the remaining ticks.
NpgsqlInterval(long) now calls NpgsqlInterval(TimeSpan) to make TimeSpan
do the division work.
Largely inspired by commit fd99912a019a3e987edfba8511431c82ec3cd9a5 from
glenebob