Create new checker for every jscs target run

Say, for example, you have this in <code>Gruntfile</code>
```js
jscs: {
    main: "main.js",
    secondary: "secondary.js"
}
```
You don't have errors in <code>main.js</code>, but you do have one in <code>secondary.js</code>.
Then, you run <code>grunt jscs</code> command without specifying the target, as a result, both targets will be run.
In stdout you get three absolutely identical errors found in <code>secondary.js</code>, instead of one.
<br/>

This would happen because
```js
jscs.registerDefaultRules();
jscs.configure( require( cfgPath ) );
```
lines will be executed twice, whereas <code>Checker</code> constructor (which is initialized from out of context of <code>jscs</code> task) saves the rules in the returned object, so they will be augment every time.

This commit resolves the problem.