Fixes #500

• Fixed issues with Pointer encoding and missing segments

openapi: 3.0.0
info:
  title: foo
  version: bar
paths:
  '/foo':
    get:
      responses:
        200: 
          description: ok
          content:
            application/json:  
              schema: asdasd

Pointer needs to look like

#/paths/~1foo/get/responses/200/content/application~1json/schema

Previously slashes were not being escaped. Also, map keys were not getting added to the path. This affected content, examples, headers, encoding

• Added better documentation to the OpenApiException.Pointer property
• Converted OpenApiException to OpenApiReaderException in parsenodes so that line number is always included if JSON Pointer is not possible.