Add convention to always declare 'use strict'

[Cruikshanks]

#115

It has been an unwritten rule for some time now that we always add 'use strict' to the top of our JavaScript files. Why?

JavaScript's strict mode is a way to opt in to a restricted variant of JavaScript, thereby implicitly opting-out of "sloppy mode".
MSDN docs - Strict mode

When the alternate mode is named 'sloppy', would you want to use it!?

The docs do provide some further explanation of what declaring strict mode means

Strict mode makes several changes to normal JavaScript semantics:

1. Eliminates some JavaScript silent errors by changing them to throw errors.
2. Fixes mistakes that make it difficult for JavaScript engines to perform optimizations: strict mode code can sometimes be made to run faster than identical code that's not strict mode.
3. Prohibits some syntax likely to be defined in future versions of ECMAScript.

For these reasons, we've had it as an unwritten convention to add it to all files for some time. This change is to 'resolve' that state of affairs and make it a written convention!

[Cruikshanks]

Eek! Ok, we don't go into detail but we already specify all files should start with a 'use strict'.

To be honest, that will do it. No need to add more words!