fix(filter.ngdoc): Check if "input" variable is defined

By default, "greeting" textfield in this example is prepopulated with "hello" text, but it's pretty easy to copy just filter code to use it in your app. If your textfield is empty while app loads, you'll get an error: "Error: [$interpolate:interr] Can't interpolate: Reverse: {{greeting|reverse}} TypeError: Cannot read property 'length' of undefined". To prevent this, we should check "input" variable, and proceed only in case it is defined.

Closes #6819.
docs($http): fix auth default headers example
docs(guides/directive): add (') to contraction


add apostrophe (') to contraction
docs(ngAnimate): change "&#64" to "@" symbol

Previously, we had problems with code that contained symbols that looked
like jsdoc directives.  This has now been fixed so we can convert these
HTML character codes back to @ signs.

Closes #6822
Closes #6826
docs(guide): add ui-router to complementary libraries
docs(guide/ie): note dropping IE8 in 1.3
docs(guide/i18n): improve content and formatting
docs(guide/ie): fix header formatting
docs(guide/e2e-testing): improve formatting and clarity
docs(guide/concepts): improve formatting and clarity
docs(guide/migration): add header
docs(README): use svg badge
docs($compile): add controllerAs example
chore: use npm cache in travis config
docs($cacheFactory): add example
chore(CONTRIBUTING): merge relevant updates from angular.dart

Back port changes to angular.dart `CONTRIBUTING.md`, as suggested by
@vicb.
docs($cacheFactory): document cache instance methods

These were apparently entirely undocumented. I'm not sure if they're intended
to be private, but in case they're not, I've written some initial docs for them
docs(errors/$injector/nomod): add info about forgetting to load the module file

Closes #3752
docs(css): Add background to .type-hint-domelement

`.type-hint-domelement` does not have a background color assigned to it.
DOM element type hints are now proudly displayed with CadetBlue.
docs(angular.bootstrap): fix param type to DOMElement