Added some initial JSDoc comments

Hi,

I started studying your escope module and began taking notes about what the different objects are about and what their features mean, to better understand how to use them. Then I realized these efforts might be better captured as API documentation.

This is just an initial pull request to check if you would be happy to receive such JSDoc comments in your code. If so I would be happy to keep on fleshing them out, and maybe you could eventually correct things I got wrong or understood poorly. The current state can be extracted into HTML documentation with 'jsdoc escope.js -r'.

T.