amTimeAgo should use angular $timeout instead of setTimeout()

[gregz67]

For better angular integration, amTimeAgo should use $timeout instead of setTimeout() directly.

https://docs.angularjs.org/api/ng/service/$timeout

[coveralls]

Coverage remained the same when pulling 2f1bb13 on gregz67:master into 83b4e33 on urish:master.

[urish]

Thank you for your contribution, it's greatly appreciated. However, using $timeout breaks e2e tests and also causes performance penalty (causes angular.js digest cycle for each update). See discussion in #35.

[gregz67]

I should have looked at past discussions. It looks like you have already explored this quite a bit. Seems like using $interval would be a good solution.