docs(Angular.js): explain that toJson strips $... properties

In Angular.toJson, any properties with a leading '$' character will be
stripped from the resulting string since angular uses this notation
internally for services.  There have been complaints of not knowing
about this functionality until it breaks within their code.
docs($cacheFactory): show that you can access existing caches
chore: add karma-script-launcher plugin

This plugin is shipped as a default one with Karma. It's specified as a peer dependency.

I assume, there's an old version of NPM on the CI server, which does not support peerDependencies and therefore it didn't get installed.

This will make the dependency explicit.
docs(guide/location): fix example code - `hashPrefix` is a method
chore(travis): speed up the build

- parallelize the tasks
- cache requests (e2e tests)

This reduces the time from ~18min to ~12min.

It makes the output little messy. We could buffer output of each task and display it once it's fully finished, nicely. I think giving instant feedback is better.
docs(tutorial/step9): formatted Unicode character line

Add tick and cross mark corresponding to their respective unicodes.
fix(jqLite): prepend array in correct order

Match jQuery behavior when prepending array into empty element
fix($compile): empty normalized href should pass sanitation check

Sometimes IE returns an empty string for its normalized href on a tags.
This should pass the sanitation check in $compile.

Closes #2219, #2593
docs(guide/directive): make directive controller minification-safe

It is best to emphasize that the "controller" property needs to be min safe

Closes #3125
docs(input): ng-model doesn't work well with isolated scope directive

Closes #3123