Improved references and a part of the introduction

I added some references--Elements of Statistical Learning and An
Introduction to Information Retrieval--which are useful books and can
come in handy during this dissertation. There were some changes to the
current bibliography: fixed some names and removed unused information.

I added a small reference to the Turing Test in the introduction, but I
should prepare a better explanation for it. There isn't a good text
around here about how language and intelligence are connected, as far as
I know, so I should write something. soon.

Also, the Makefile reflects a better organization, as in the default
task is cleaning up after generating the new PDF and the clobber/clean
tasks are working correctly. Unfortunately, `-aux-directory` is only
available in MikTeX... :(