Allow routes other than /blog (2-0-stable)

Hello!
This gem is nice and I used it on one of my projects. I had to change URLs for routes from 'blog' to 'news'. I want to share my implementation as I saw similar task in todo list.
I've added a new preference to Spree::Config. It's called 'blog_alias'.  In case of running Spree::Config[:blog_alias] = 'news', routes table will be as follows:
```
           blog_entries   GET    /news(.:format)                                     spree/blog_entries#index
                          POST   /news(.:format)                                     spree/blog_entries#create
         new_blog_entry   GET    /news/new(.:format)                                 spree/blog_entries#new
        edit_blog_entry   GET    /news/:id/edit(.:format)                            spree/blog_entries#edit
             blog_entry   GET    /news/:id(.:format)                                 spree/blog_entries#show
                          PUT    /news/:id(.:format)                                 spree/blog_entries#update
                          DELETE /news/:id(.:format)                                 spree/blog_entries#destroy
               blog_tag          /news/tag/:tag(.:format)                            spree/blog_entries#tag
          blog_category          /news/category/:category(.:format)                  spree/blog_entries#category
            blog_author          /news/author/:author(.:format)                      spree/blog_entries#author
   blog_entry_permalink          /news/:year/:month/:day/:slug(.:format)             spree/blog_entries#show
           blog_archive          /news/archive/:year(/:month)(/:day)(.:format)       spree/blog_entries#archive {:year=>/(19|20)\d{2}/, :month=>/[01]?\d/, :day=>/[0-3]?\d/}
              blog_feed          /news/feed(.:format)                                spree/blog_entries#feed {:format=>:rss}
                   blog          /news(.:format)                                     spree/blog_entries#index
```
In such a way helpers will always be unchanged and have prefix 'blog', and URl can be changed.
8 tests are failed now. I can fix these tests next time :)