Long exact sequence of homotopy groups + Generator of π₃S²

[aljungstrom]

This PR contains

1. The long exact sequence of homotopy groups (and corresponding loop spaces). It also contains some information about how the maps in the sequence look when they are transported to the (Sⁿ→∙A) definition of homotopy groups. Right now exactness is stated in the usual way by having a bunch of separate theorems on the form Ker⊂Im and Im⊂Ker, like in Mayer-Vietrois and Gysin. Anders and I have been discussing adding a reasonable definition of exact sequences for some time now. When that's a reality, things will look nicer.
2. A proof of π₃S²≅ℤ together with the fact that the Hopf map is the generator of π₃S².
3. A proof of πₙSⁿ≅ℤ together with the fact that the identity map is the generator of πₙSⁿ (needed above). The construction of the Iso is somewhat roundabout to make the characterisation of the generator easier. The characterisation of the generator is longish for π₂S² with, but easy in all other cases. Hopefully in the future this step will hold simply by computation.

[aljungstrom]

@mortberg Ready!