refactor(angular.js): improve trim performance

According to Flagrant Badassery's blog
http://blog.stevenlevithan.com/archives/faster-trim-javascript
and this comparison http://jsperf.com/trim-function, this trim method is faster.

Closes #4406
docs(guide/scope): access the current element's scope in the console.

Closes #4884
docs(CONTRIBUTING): fix broken link to GitHub PR Helper

Closes #5134