Adjustment for scoping

The code was wrapped in two anonymous functions, which could cause `this` to be undefined depending on how the loglevel code was included in a project. 

Only using one anonymous function and having this be at the "top-level" allows the code to be wrapped and invoked with apply like so:

```javascript
var myScope = {}:
(function(){
 // loglevel.js source
 // "this" will equal myScope, since the "this" in loglevel.js is in this scope.
}).apply(myScope);
myScope.log.debug("log was defined on myScope");
```

The code change follows the same pattern as this:
https://github.com/marionettejs/backbone.marionette/blob/master/src/build/marionette.core.js

I'd love to write a unit test but I'd have to adjust the `Gruntfile.js` and include grunt-preprocess or grunt-rigger, which are code injection/include tools, to generate a test that includes the `loglevel.js` wrapped in a function invoked with apply. If you want me to do this I will. 

I also hope that my change doesn't affect anything that the `undefined` argument was doing.
https://github.com/pimterry/loglevel/blob/master/lib/loglevel.js#L8

Thanks,
Gianni
I see what you're saying, but isn't the "support" of QUnit just not overwriting the `log` method? We're being too specific. We can support all versions of QUnit, Mocha, Jasmine, anything, as long as we don't change things in the global context. 

If I'm missing something let me know. To me it seems that `noConflict` is what we're testing. Other libraries, backbone, underscore, don't test for QUnit support, they just test that `noConflict` restores the original value.