Correct the example given for a future, add punctuation where necessary
Add a small section on futures to the tutorial
Remove trailing whitespaces
auto merge of #6565 : osaut/rust/futures, r=thestinger

* The example given in future.rs was corrected.

* I have added a small section describing futures in tutorial on tasks. It is far from being complete as I am stil learning !

(This is an updated version of PR 6537).