Made it up-to-date with chapter 11
Second version.

Added:
* the correct definition of formal moduli problem;
* a coherent discussion on the concept of infinitesimal extension;
* the definition of the tangent spectrum of a formal moduli problem;
* more evidence for the main theorem.

Still to do:
* the example of Hochschild cohomology;
* discussion on cdga and dgla (should be done by Brice) and
Maurer-Cartan;
* the sketch of the proof of the main theorem.
Minor changes

Fixed some small typos.