Add jshint validthis:true directive

JSHint complains about the use of `this` inside the main plugin function, saying "Possible strict mode violation", because it's not clear that this function is going to have a specific context bound to it.

You could add `"validthis": true` in the `jshintrc`, but seeing as the issue is specific to this function definition, I suggest just adding it inline. (NB. you can't do `/*jshint validthis:true*/` at the top of a file; interestingly it [only works inside a function](http://jslinterrors.com/option-validthis-cant-be-used-in-a-global-scope/).)