add support for bower

regarding PR #30 I might be more confortable using bower to manage the assets instead of a third party. 

With this PR, you just need to get this in the root of your app: 

```json
{
  "directory": "public/vendor/"
}
```

then you add your front end assets in bower.json like you'd in your Gemfile:

```json
{
  "name": "project",
  "version": "0.0.1",
  "private": "true",
  "dependencies": {
    "jquery": "~> 1.11.0",
    "bootstrap": "3.1.1",
    "font-awesome": "4.0.3",
    "modernizr": "git@github.com:Modernizr/Modernizr.git",
  }
}

```

then ```bower install```will install all the libs with dependencies and sources etc.

You can now require them in your scss:
```scss
@import 'bootstrap';
```
and js:
```js
//= require 'bootstrap'
```

How do you guys like it? @did @jaredklewis