Added option to only copy the packages that are declared in bower.json:depedencies

Hi,

i added a new option "onlyDependencies" so that when "copy" is set to true, it only takes the packages declared in `bower.json:depedencies`.

For example if you want to use only bootstrap css and fonts but not its javascript, in the old version you could declare something like this in **bower.js**:

```json

  "exportsOverride": {
    "bootstrap": {
      "css": "dist/css/*.*",
      "fonts": "dist/fonts/*.*"
    }
  }
```
but it will copy the jquery depedency because bower installs it in bower_components as it's a declared depedency of bootstrap.

so by adding the **onlyDependencies** property in the task declaration in **gruntfile.js**:

```javascript

        bower: {
            install: {
                options: {
                    targetDir: "./wwwroot/lib",
                    layout: "byComponent",
                    onlyDependencies: true,
                    bowerOptions: {
                        production: true
                    }
                }
            }
```
it'll only install the css and fonts of bootstrap.