Add new option `expand`

An option for organizing the file structure by package rather than placing them all in one directory. Useful when some filenames could intersect with each other.

```javascript
bower: {
  dev: {
    dest: 'public/vendor/',
    options: {
      expand: true
    }
  }
}
```
```
/public
  /vendor
    /package1
	  package1_file1.js
	  package1_file2.js
	  package1.css
	/package2
	  package2.js
	  package2.css
```

Or organized by file type in addition:
	
```javascript
bower: {
  dev: {
    dest: 'public/',
	js_dest: 'public/js/'
	css_dest: 'public/css/',
    options: {
      expand: true
    }
  }
}
```
```
/public
  /js
    /package1
	  package1_file1.js
	  package1_file2.js
	/package2
	  package2.js
  /css
    /package1
	  package1.css
	/package2
	  package2.css
```