Fix #1, #9 and #29. Add ignores support.

This pull request add `.jshintignore` support to the jshint task config. Since https://github.com/jshint/jshint/pull/949 has been merged upstream into jshint, this is a relatively simple change. 

In order to load the default the `.jshintignore` from a projects root there is no config change needed. It will be loaded by default.

Should you want to define custom ignores you can use the `ignores` option. This is an array where each element represents a line of the `.jshintignore` files. An example config would look like:

```javascript
    grunt.initConfig({
        jshint: {
            all: ['*.js'],
            options: {
                jshintrc: '.jshintrc',
                ignores: [
                    'lib/',
                    'path/to/3rdparty/lib.js'
                ]
            }
        }
    });
```