Calculator: fits on Emulator screen

* Horizonally centered & width limited to 550px
* Vertically limited to 665px, but not centered because of Emulator quirk
* uses CSS flex-boxes, because this app will not be needed on any non-webkit platforms

Open-WebOS-DCO-1.0-Signed-Off-By: [Doug Reeder] <reeder_29@yahoo.com>