Calculator: centered
* Horizonally centered & width limited to 550px
* Vertically centered & height limited to 665px
* Uses CSS flex-boxes, as this app only needs to run on WebKit.

Open-WebOS-DCO-1.0-Signed-Off-By: [Doug Reeder] <reeder_29@yahoo.com>
Merge pull request #4 from DougReeder/master

Calculator: centered