Fix conflict between +bootstrap and +site features.

When I generate a luminus project with `lein new luminus NAME +site` everything works fine,
but when I use `lein new luminus NAME +bootstrap +site`  (let's imagine user don't know that `+site` feature will add boostrap automatically) I get css, js duplications in my `layout.clj`:
```
(defhtml
  base
  [& content]
  [:head
   [:title "Welcome to meteor"]
   (include-css
     "/css/bootstrap.min.css"
     "/css/bootstrap-responsive.min.css"
     "/css/bootstrap.min.css"
     "/css/bootstrap-responsive.min.css"
     "/css/screen.css")
   (include-js
     "//ajax.googleapis.com/ajax/libs/jquery/1.9.0/jquery.min.js"
     "/js/bootstrap.min.js"
     "//ajax.googleapis.com/ajax/libs/jquery/1.8.0/jquery.min.js"
     "/js/bootstrap.min.js")]
  [:body content])
```

As I understand it happend, because `site/layout.clj` already has these dependencies https://github.com/yogthos/luminus-template/blob/master/src/leiningen/new/luminus/site/layout.clj#L32 and extra dependencies was added by `post_process :+bootstrap`.

To fix this behaviour I've removed dependencies from `site/layout.clj` and added "+bootstrap" to the `features` atom in the `add-feature :+site` method. Looks like it should work in both cases now, when features are used together or separately. 

If it sounds right, you could merge it.