Further update

More than half way on homotopy (co)limits. What's new:
* added several motivations and intuitive explanations about homotopy
(co)limits;
* added the existence of the global projective model structure for
diagrams over a cofibrantly generated model category;
* added a big remark concerning the global injective model structure.

What's missing:
* the simplified criteria for homotopy pushout and homotopy pullbacks;
* computations of loops and suspensions;
* Bousfield-Kan formulas (just the formulas, not the proof that they do
what we want).