Updated CLLocationManagerDelegate method for locationManager:didUpdateLocations

Updated `CLLocationManagerDelegate` methods for `locationManager:didUpdateLocations`

*Changed `locationManager:didUpdateToLocation:fromLocation` to `locationManager:didUpdateLocations`
*Preserved callback `result[:from]` key-value by storing previous location in an instance variable
*Added `result[:previous]` to include additional locations returned by the delegate method
*Updated inline documentation for `location.rb`
*Updated spec `BW::Location`
*Updated `README.md` with information about 'result[:previous]' value

Updated RSpec tests passed, tested on iOS Simulator 8.0 (iPhone 6) and iPhone 4S (iOS 8.0.2) device, compiled with RubyMotion 2.34.