Milliseconds

I ran into the case in which Chronic was only able to parse the string format if the milliseconds didn't contain 3 digits

example: 
`"2013-08-01T19:30:00.34-07:00"` would return nil
`"2013-08-01T19:30:00.340-07:00"` would return 2013-08-01 22:30:00 -0400

https://github.com/mojombo/chronic/issues/242