I really don't like to pollute every single file of JsDuck with
GPL license header as the GNU guys suggest.  So I just include it
into the main jsduck.rb file.  No harm done, I hope.

Plus some re-arrangements in README.

Plus some ExtJS strangenesses in use of @member.

Not quite correct, but for now it's better for them to behave the same.

Again one of the most used @tags in ExtJS.

Only supporting it in class declaration, plus some explicit logic to
detect it when it appears after @constructor in class doc-comment.

Applies only to properties and methods.

This was one of the most used @tags in ExtJS not already supported by
JsDuck.

The most important at-tags are now supported.