dropCwd option, fixes #15

```javascript
filerev: {
    options: {
        dropCwd: true
    },
    css: {
        expand: true,
        cwd: 'public',
        src: ['css/main.css'],
        dest: 'public'
}
```
would result:
```javascript
{ '/css/main.css': '/css/main.4458448b.css' }
```
instead of:
```javascript
{ 'public/css/main.css': 'public/css/main.4458448b.css' }
```
If you want to drop heading `/` as well `cwd` has to be specified as `public/`