Removed reference to obsolete comment system
Default INSTALLED_APPS contains other apps now.

Have added messages and staticfiles app to INSTALLED_APPS example, since they
are now included in the tuple by default.

Also, since the book does not deal with either of these, have changed the
wording slightly to emphasize that not all apps will be discussed later.
Url pattern now using url() for consistency
Merge pull request #29 from stooj/master

Removed suggestion of using the comments system from the introduction