Support namespace description in swagger doc

[renier]

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:

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.

[renier]

An example. Adding this namespace description from grape-->grape-swagger is not possible as-is, however it is allowed in the swagger spec. This is what the pull-request enables.

[dblock]

Merged via tim-vandecasteele@df82041. Please do re-review, there were some changes, notably around the default description value.