remove local iso8601 and use pypi version

When running some tests on another project I stumbled upon a failed
test that revealed a bug in colander's iso8601.  If you try to
deserialize "2014-09-09T15:15:57.516967" into a datetime object you
get `datetime.datetime(2014, 9, 9, 15, 15, 57, 516966)` (notice the
microseconds are off by 1).  I traced it down to this:

```
groups["fraction"] = int(float("0.%s" % groups["fraction"]) * 1e6)
```

`int(float("0.%s" % "516967") * 1e6)` is equal to 516966 due to
rounding.

I was going to fix it, but iso8601 is a currently maintained project
on pypi that has this issue (and possibly others) already fixed.
So, it makes much more sense to offload everything back to that project.

The only difference (besides the bug fixes) is following:

`iso8601.Utc` => `iso8601.iso8601.Utc`
`iso8601.FixedOffset` => `iso8601.iso8601.FixedOffset`