Minor update

Efficiency...
Better error management

using `.catchError` is a good practice to cacth errors throws during the `then` handler.
Add img/ directory for screenshots.

BUG=

Review URL: https://codereview.appspot.com/55310043
Tweaks to Chapter 1

Review URL: https://codereview.appspot.com/55400043
Fix some typos.
Fix syntax error.
avoid duplicate ids in ng-repeated span (category-filter)

Eliminated the need for the <span> id by putting the span inside the
<label>.
unnecessary test for null removed from CategoryFilter.call

No need to test for (filterMap != null) immediately before the conjunct
(filterMap is Map) since (null is Map) is false.
Remove value attribute of text input having ng-model (Ch.5)

Given that the “name-filter” text input is bound to the
ctrl.nameFilterString model, the presence of the input value attribute
seems at best useless, but is more likely to be confusing to beginners.
Also removed input end tag (since input is an empty / void element).
Merge pull request #44 from chalin/patch-3

Remove value attribute of text input having ng-model (Ch.5)
Merge pull request #36 from bgourlie/patch-1

Fix some typos.
fix(Chapter 7): fixed parser gen header file
Merge pull request #45 from pavelgj/master

fix(Chapter 7): fixed parser gen header file
Use NgAttrs instead of map

Review URL: https://codereview.appspot.com/53960043
Merge pull request #39 from chalin/patch-1

avoid duplicate ids in ng-repeated span (category-filter)
Merge pull request #40 from chalin/patch-2-unneeded-test-for-null

unnecessary test for null removed from CategoryFilter.call (Chap. 5-7)
Cleaned up imports (unused and missing) and tweaked test (Ch.2 to 6)

 - Cleaned up imports, unused and missing (Ch.2 to 6).
 - Adjusted called to Recipe constructor in main_test (Ch.5&6).
 - Chose not to touch Chapter 7 code given that there are other pending
changes for it.
Merge pull request #46 from chalin/patch-4-mainly-import-cleanup

Cleaned up imports (unused and missing) and tweaked test (Ch.2 to 6)
Merge pull request #37 from bgourlie/patch-2

Fix syntax error.
Add screenshot for ch02

Review URL: https://codereview.appspot.com/57940043