How do I setup this Exclusion Constraint?

I would like to prevent overlapping dates ranges for col1 + col2 from
being inserted into my test table.

Existing Data:
1, FOO, 2012-04-04, 2012-04-06

Insert Attempts:
1, FOO, 2012-04-05, 2012-04-08 <-- BAD, overlaps w/ above!
1, BAR, 2012-04-04, 2012-04-06 <-- OK, no conflict!
2, FOO, 2012-04-04, 2012-04-06 <-- OK, no conflict!

Here's the table:

CREATE TABLE test (
id INTEGER NOT NULL DEFAULT nextval('test_id_seq'),
col1 INTEGER,
col2 VARCHAR(10),
from_ts TIMESTAMPTZ,
to_ts TIMESTAMPTZ,
CHECK ( from_ts < to_ts )
);

I'm trying to used what I learned in
http://www.depesz.com/2010/01/03/waiting-for-8-5-exclusion-constraints/,
but I cannot figure out how to apply this exclusion constraint to col1
(integer) + col2 (varchar).

Also, I'm very new to postgresql, so if you could explain it, that'd
be great too. And must I compile postgresql from source to gain the
ability to use this type of exclusion constraint?