Added onChange method to options

[jezzdk]

Just added a simple onChange option, as I needed it in my project.

[abpetkov]

Few things:

• dist/ is not the proper place to add those changes to. You should use switchery.js instead. The files in the dist/ folder are auto-generated - check out the Readme to find how to do this.
• this should better go in handleOnchange() method
• fix your indentation
• onChange option could be renamed to something like changeCallback

First 3 are strong advices, can't merge this without it happening the proper way. The last - I'm not really sure about it, I'm trying to come up with a better name, onChange just doesn't sound right for some reason, feel free to change this to something even better.