Exercises 3.4 and 3.5

If there is a shorter solution to 3.4, then I'd like to know (and will add/substitute).

> How about `apply ap10, path_contr`?
>
Very good. I was looking for "happly" in the library, and missed that  
guy "ap10".
"contr" works also great ("center" not explicitly needed).

See the new commit. There is some magic going on in that coq  
understands that IsHProp A implies IsHProp (A->A).

Thanks,

Marc