In the novice shell lesson 03-pipefilter, changed sort to sort -n. In the lesson , the "sort" command is used to demonstrate sorting line counts. The lesson as written does not produce the given output on the bash shell (but does on Mac OS X terminal). By adding the -n to sort, the example should behave the same across all platforms.
Fix issue when building book

When try

    $ git clean -xdf
    $ make site
    $ make book

the HTML version of book will not be create.

This problem is fixed adding a few new rules.
Add title to book

This will help when converting to EPUB (and probably PDF) and also
improve the text at web browsers top bar or tab.
Update template for book to rely on CSS

Use h2 tag for "subtitle" will break any tool used to convert HTML
for another format. This commit replace h2 with span and rely on
CSS for have the same look.
Merge pull request #538 from mestato/fixing-sort-in-shell-lesson-3

In the novice shell lesson 03-pipefilter, changed sort to sort -n.
Merge pull request #539 from r-gaia-cs/fix-book-make

Fix issue when building book
Merge pull request #540 from r-gaia-cs/add-title-to-book

Add title to book
Merge pull request #541 from r-gaia-cs/book-css

Update template for book to rely on CSS