Notify send custom images

[flga]

This change allows the user to customize the notification image on Linux.

Path resolving is done by node's path.resolve(to).
Relative paths are admissible.

Example:

//in node
var path = require('path');
var notify = require('../lib/notify-lib');

notify({
    image: 'my_logo.png',
    duration: 4,
    title: '...',
    message: '...',
});

//in grunt
grunt.initConfig({
    notify: {
        options: {
            duration: 4,
            image: 'my_logo.png',
            title: '...',
            message: '...',
        }
    }
});

[honsa]

The commit looks good, just one condition added, if option.image is set use the string to the path otherwise the default image path is used.

Please approve this pull request :)