[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.

closes #2098
@tdas I renamed "filters" to "track" in the new methods so it matches the name that FilterQuery uses. The name "filters" for the keyword set doesn't make much sense anyways, since locations and follow are also filters.

Is it ok to rename "filters" in the original methods too?