Added cl-postgres integration.

In the same vein as the cl-postgres+local-time system this adds a new
system, cl-postgres+local-time-duration and exports a
set-local-time-duration-cl-postgres-reader funtion which install a
reader converting postgres intervals to local-time-duration instances.
The method on cl-postgres:to-sql-string is always defined.
Make sure output generated in the print-object method always goes to the right stream.

There was an implicit reliance on being able to pass nil as a stream and
have the string output simply be returned; in the case of print this is
not true. Also a duration with non-zero nsecs should not be considered
as an empty duration. Made stream argument to human-readable-duration
required.
Call a duration of length 0 (which does occur in practice) a 0 length
duration instead of an 'empty duration.'

the reasoning being that if someone has explicitly called (duration :sec
0) we don't want to hide that information
Merge pull request #1 from segv/cl_postgres_integration

cl-postgres integration