Encapsulates info from server.rb into Weather class

 - moves hash of current weather information into the Weather class,
   from the server.rb file
 - creates the current_weather_hash method, which takes the xml
   data structure as input
 - traverses the xml file in different ways to pull out all needed info
   (since the info needed is in different places in XML tree)
 - saves each piece of info into a variable
 - variable then saved as value in new key/value pairs in the returned hash