fix(ngSanitize): sanitizer should not accept <!--> as a valid comment

According to http://validator.w3.org/ , <!--> is not a valid comment
and neither is any comment containing the -- substring.
test($browser): correct false positive in ApplicationSpec.js

Previously, the check that Application should return a new $window and
$document had the arguments reversed in the first call to navigateTo;
thus, the subsequent check of inequality of $window and $document in the
next navigateTo call would always pass.

This corrects the argument order, which makes this test not succeptible
to false positives.
docs(guide/e2e_testing): clarify that .enter searches by ng-model
fix(ngOptions): ignore object properties which start with $
docs($http): explain why $http may not make the request immediately

I came across this issue today and after researching has found out this thread on so:
http://stackoverflow.com/questions/17039998/angular-not-making-http-requests-immediately.

It took me quite sometimes to figure out this so I hope the addition in documentation could save somebody else some times and frustration.
docs($http): add examples when calling $http outside $apply

Closes #3996
docs(ngController): rephrased the description for clarity
docs($browser): add jsdoc tags and fix typo
docs(ngSwitch): fix minor typo

Closes #3993
fix($sce): make trustAs watchable by caching its instances by type

Closes #3932
Closes #3980
style($sce): remove unused function and add missing semi-colons
docs($sce): fix typo