Validate Time fraction for date-time

As per http://www.faqs.org/rfcs/rfc3339.html, date-time can include a time fraction.
That means that `2014-09-05T20:19:28.99Z` is a valid date-time for json_schema.  

I adapted the regular expression to reflect this.
`time-secfrac    = "." 1*DIGIT` is the ABNF definition and allows as many decimals as we want.

I followed the definition but practically I guess we could `{1,3}` the time fraction digits part.