Modularise the .js logic

The .js file at

https://github.com/cyclestreets/telluswhere/blob/master/js/telluswhere.js

contains a mixture of functionality, which ideally should be modularised.

For instance, the icon layer functionality ought to be separate to the setting.

http://stackoverflow.com/questions/22175013/how-do-i-refactor-out-one-function-from-a-javascript-module-to-another-js-file
