Possible improvement: add the ability to exclude the end of the interval when splitting by period

[vladimir-gavrilenko]

Hi!

According to #83 and #79 the end of the interval is included when splitting by period. I guess there should be a way in this library to cover not only the inclusive case but also the exclusive.

To cover the exclusive case I suggest one of the following:

1. Add until to RichReadableInstant.scala

// RichReadableInstant.scala
// ...
def until(other: ReadableInstant): Interval = to(other).withEnd(other.toInstant.minus(1L))
// ...

// User's code
val inclusive = start to stop by 5.minutes
val exclusive = start until stop by 5.minutes

The problem here is that interval contains interval.end is false.

2. Add exclusiveBy to RichInterval.scala

// RichInterval.scala
// ...
def exclusiveBy(period: ReadablePeriod): List[DateTime] = by(period).dropRight(1)
// ...

// User's code
val inclusive = start to stop by 5.minutes
val exclusive = start to stop exclusiveBy 5.minutes

I will be glad to create a PR with one of the proposed or any other solutions.