[Fix #158] Use emoji to display green checkmark.
Fix non-checkmark cell indentation.  Reuse image.

Regression from 73e6eba32bf4d09fb07b2061ebf63758ebde2969.
[Fix #160] Check which textfield's contents is being set as the NSTimer's userInfo parameter during the sensor walkthrough.
Merge branch 'Development' of https://github.com/NIST-BWS/wsabi2 into Greg
Add some primitive logging for unexpected exceptions and some signals.