Implement GetCaptions
Implement GetCaptions

GetCaptions retrieves the captions (text only) for the given videoid,
track name and language, available tracks can be found with CaptionSearch

Minor changes to the other search classes to accommodate how GetCaptions works,
as we can no longer include max-results, start-index or api version (v) by
default, v in timedtext indicates the video id which would always be replaced
by the api version.

Also changed __setTotalResults in Search class as timedtext doesn't provide
this.

WillBickerstaff/getcaptions