Add "flex" to layout's package.js

Adjust flex sample to fit into Enyo Sampler.
Make code all jsHint happy, use "smart tabs" where lines can start with tab characters, but won't use them internally.
Removed stripping of "px" since parseInt stops at first non-number.
Constrain sample CSS to new flex-layout-sample class
Add enyo-fit class to sample to fix height when running in panel as opposed to document.body
Remove unused duck asset

Enyo-DCO-1.1-Signed-Off-By: Ben Combee (ben.combee@lge.com)