Replacing literal strings with constant strings

How would you feel about replacing the literal strings with constants?

I think it will make the code easier to understand, and to maintain.

I thought to do this because I ran across a difference in app.js, and brain.js. App.js uses the string 'scan_address' to refer to the section of HTML to display. It then tells brain.js to 'start', and then brain.js uses the string 'scanAddress' to indicate its state. There's no reason the two strings should be different; they can both use 'scan_address' and achieve the same effect.

I'm sure there are other similar cases.. all that would be avoided by using constants, among other benefits.

This pull request only changes a few strings to constants, to give an idea what I plan to do. If accepted, I will make the necessary changes elsewhere, too.