Chapters 1-3: Created content of new chapter 2 out of old chapters 1 and 2 (now 3):
Chapters 1-3: Created content of new chapter 2 out of old chapters 1 and 2 (now 3):

2.1 NLTK Corpora (was 1.5)
2.2 More Python: Functions (was 1.4.3)
2.3 Semantic Networks (was 1.7)
2.4 More Python: Getting Organized (was 1.6)
2.5 Accessing Text on the Web (was 2.1)
2.6 Strings and Files (was 2.2)

Moved discussion of not using keywords for variable names to chapter 1, since
earlier discussion was postponed along with all the other material on strings.

Chapter 4: opening example of text.similar() included punctuation
in the output.  Modified nltk/text.py to exclude punctation tokens
from ngrams, and updated examples in chapter 4.

Misc polishing, added a couple of exercises.


svn/trunk@6641

nltk/nltk,nltk/nltk,nltk/nltk