docs($injector): add example on how to use the element.injector

Closes #5188
chore(CHANGELOG): introduce perf() section for performance-related commits

Instead of using fix() or chore() when labelling a commit which improves
speed or performance use perf(). Perf commits will be listed in the
CHANGELOG under "Performance Improvements".

For example:
perf($animate): cache all getComputedStyle operations to reduce additional reflows
Merge branch 'master' of https://github.com/angular/angular.js
componentizd!