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
docs(guide/expression): add section on $event
docs(ngEventDirs): link to info on $event

Closes #6724
docs(tutorial/step_02): fix typo