don't store the iterated tweets to save memory space

Hey, @sferik 
I found when I searched for massive tweets ,I got my memory bloated. the reason is we always store and reference the tweets in our @collection array.
And the old used , or iterated ones, seems no use case in our code ,and I think storing the objects is the gem users' responsibility. 
This is one performance which will affect some users like me.
What do you think? 
will be good if we can merge this PR.
thanks.