Added support for operationId on endpoints.

[Tetha]

Hey,

I've been tinkering around with this gem and a small sinatra application. However, when I wanted to use the swagger-js client with my backend, I learned that swagger-js uses the operationIDs in the API to generate the javascript methods on the client. Sadly, this gem doesn't support that out of the box already.

So, I've added some things:

• a method endpoint_operation_id and corresponding argument to endpoint. This is pretty much identical to summary or description.
• I've added tests similar to description and summary.
• I've documented the integration between swagger-js and sinatra-swagger-exposer, because I had to search around some to get this to work.