Support namespace description in swagger doc

Grape does not support attaching a description to a Namespace. It always flows to the next Route. So when Grape-Swagger rolls the routes up and groups them per path (i.e. namespace), there is no description to attach to the overall namespace.

This pull request allows you to say:
```ruby
namespace :hello, desc: "Group of APIs to say hello" do
  ...
end
```
`namespace` is an alias for other methods like `resource` and `group` in Grape.

The generated swagger doc json will then contain the namespace description, and the swagger-ui will naturally add it to the api heading.