[SPARK-2788] [STREAMING] Add location filtering to Twitter streams

TwitterUtils.createStream(...) allows users to specify keywords that restrict the tweets that are returned. This change adds a location parameter that also restricts the returned tweets.
I should have caught and commented on this earlier, but why is this `Seq[Seq[Double]]` and not of `Seq[(Double, Double)]` ? Its not like that the location will ever be a sequence of more two doubles. So having a Seq[Double] for latitude and longitude is pretty confusing. In fact having (Double, Double) is still confusing, as it is not obvious which one is latitude and which one is longitude. Hence, i think that its best to define a `case class Location(latitude: Double, longitude: Double)` (within the `org.apache.spark.streaming.twitter` package), and use that. This should be most intuitive and least ambiguous.

What do you think?