arXiv:1201.2991v1; In essence, this paper consists of the notes of four lectures delivered in May 2011 as part of the Chaire de la Vall'ee Poussin 2011 <http://www.uclouvain.be/15551.html>. The third and fourth lectures were also part of the conference Category Theory, Algebra and Geometry on 26 and 27 May 2011 in Louvain-la-Neuve, Belgium