Updated date-time regex to accept zero or one of : in the timezone designator

Hi Kenny,

I've updated the date-time regex to accept zero or one of ":" in the timezone. I've noticed that the ISO-8601 standard states (http://www.w3.org/TR/NOTE-datetime) that there should always be a colon but have found that in other sources that sometimes the colon doesn't appear. Please see the sources below:

1) http://en.wikipedia.org/wiki/ISO_8601 - see time zone designators section
2) http://www.cl.cam.ac.uk/~mgk25/iso-time.html - see timezone section

I've added a few tests within test_jsonschema_draft4.rb and removed a test which no longer fails in test_jsonschema_draft3.rb.

Let me know if you want anything else added? If you don't want to merge this PR for any reason no worries, if you could let me know why (just out of interest) that would be great.

Regards,
Jon