Date() does not validate Unix timestamp with fractional seconds (decimals)

Timestamps generated with fractional seconds (i.e. `date +%s.%N` => `1451427344.877632802`) do not validate as dates.

This appears to be a result of `Hoek.isInteger(value)` on [/lib/date.js#L61](https://github.com/hapijs/joi/blob/master/lib/date.js#L61).
