Use 'attached' for deprecated 'enteredView' and 'detached' etc...

Just working through some example code and I couldn't get the 'enteredView' function to fire so found that it had been replaced by 'attached' in my version of Polymer 0.5.1.  Similarly for 'removed' to 'detached'

core-component-page#0.5.1 bower_components/core-component-page
├── polymer#0.5.1
└── webcomponentsjs#0.5.1

(Attributed : https://www.dartlang.org/docs/tutorials/polymer-intro/)