adds some tests
fixes terrible bug in updateTimeIter:
	we checked missingTweets.containsKey( )
	with a TWEET parameter instaed of STRING (ID) parameter!!
	
	same problem in in missingTweets.remove( )
	
	god bless testing
new Exception classes : TweetAlreadyExists , TweetDoesNotExist

adds some throws to the methods add(TweetAlreadyExists) , combine
(derived because it uses add) and getTweetLifeTime (TweetDoesNotExist)
added many TODOs about next to come tests and fixes, matching what i
read in the piazza (i read it all)
fixes a bug caused by throwing an exception in method add
put getSummeries in comment, it seems to be redundant now.
more shitload of tests, only left to add global tests...
meaning system level tests

i am very satisfied with the unit test for TweetDict, it covers lots of
cases ;)

good morning!