Make (cl-postgres:to-sql-string duration) not return a second value so s-sql won't try to escape the returned string.

Also change the code to generate the sql interval expression to be a bit
easier to read.
New function decode-duration with similar api and usage as local-time:decode-timestring.

Use this function to implement human-readable-duration.
Added format-iso8601-duration; use it to implement (cl-postgres:to-sql-string duration)
Make sure to properly quote and cast (in sql) the output of (cl-postgres:to-sql-string duration)
Move all the format code, and utils, into format.lisp (new file)
Merge pull request #3 from segv/master

implement format-iso8601-duration and decode-duration