Add millis method to RichDateTime

[kmizu]

• See millis in RichDateTime #61

[xuwei-k]

please squash commits.

[kmizu]

@xuwei-k OK.

[xuwei-k]

LGTM

[rtyley]

This actually broke some code for me!

We used to have:

val t: Long = expiration.millis

...which was using com.github.nscala_time.time.RichReadableInstant in nscala 0.8.0:

def millis: Long = underlying.getMillis

... but the new code in com.github.nscala_time.time.RichDateTime with this PR (def millis: DateTime.Property = underlying.millisOfSecond()) clashed with that (because we were using import com.github.nscala_time.time.Imports._) and caused a compilation error:

type mismatch;
[error]  found   : org.joda.time.DateTime.Property
[error]  required: Long

Not saying that this PR shouldn't have gone in, just surprising how things can break, is all...

As a cheap fix, I'm now just importing:

import com.github.nscala_time.time.Imports.richReadableInstant

[rtyley]

Or, better, just expiration.getMillis...!