While this is a first step in resolving grpc-ecosystem#128, this
needs to be cleaned up, and the same approach needs to be used for
messages, message fields, et al.

echo_service.proto has been annotated with extra comments in order to
demo the new descriptions.

Only the Swagger example has been regenerated, as my local generator
does not output all the expected fields in proto struct tags.

…essages using proto comments

Also, fixed a few typos and renamed protoPath into protoPathIndex
(as we are not returning the entire path in that method).

This field should generally be non-nil while running the generator.
This commit also adds a trivial check to make panic more informative
in case a future test breaks in a similar fashion.

This patch will extract summary separately from description in
Swagger objects that happen to support both summary and
description. It will do so by splitting the relevant proto
comment based on paragraphs, and using the first paragraph
as the summary.

This patch will also allow updating the Swagger schema's
otherwise hard-to-map fields by allowing custom JSON to be
placed within the proto comment. The syntax leaves something to
be desired, but it works. Use of JSON is, on the other hand,
rather universal.

This patch will also allow describing the whole API by attaching
the comments to the 'package' stanza inside the proto. In this
case, summary will be applied to info.title, and description
will be applied to info.description. Other properties have to be
set through JSON.

Schema has been slightly updated to allow for fields such as
termsOfService, contact, license, externalDocs, etc.

If a method is not documented, stub summary for Swagger
Operation is no longer generated.

Documentation for enum values is now multiline with dashes
before values.

While the spec does not seem to dictate this, most of the
examples seem to have HTTP status codes as the keys. This
patch replaces the text 'default' with value of '200'.

[1]: grpc-ecosystem#134 (comment)