Modified gem to allow page number rendering in reply_to links

Added @post_positions to topics show action:

```ruby
def show
  if find_topic
    register_view
    @posts = @topic.posts
    unless forem_admin_or_moderator?(@forum)
      @posts = @posts.approved_or_pending_review_for(forem_user)
    end
    @posts = @posts.page(params[:page]).per(Forem.per_page)
    #Provides ids of posts and their positions
    post_positions = @topic.posts.flatten.collect{|p| p.id}.each_with_index.to_a
    @post_positions = post_positions.map{|a| {:id=>a[0], :position=>a[1]+1}}
  end
end
```

Modified posts/_post.html.erb view:
```erb
<% if post.reply_to %>
  <span class='in_reply_to'>
    <% position = @post_positions.select{|p| p[:id].equal? post.reply_to.id}.first[:position] %>
    <% page = Integer(Float(position-1)/Float(Forem.per_page)+1) %>
    <%= link_to "#{t("forem.post.in_reply_to")} #{post.reply_to.user}", "?page=#{page}#post-#{post.reply_to.id}" %>
  </span>
<% end %>
```

Modified topics/show.html.erb to pass data to partial:

```erb
<div id='posts'>
  <%= render :partial => "forem/posts/post", :collection => @posts, :locals => {:post_positions=>@post_positions} %>
</div>
```